documentclass{scrartcl}
usepackage[T1]{fontenc}
usepackage[utf8]{inputenc}
usepackage{scrlayer-scrpage, scrlayer-notecolumn, lmodern, blindtext,
ragged2e}

%% step 1: counter
newcounter{snmark}
setcounter{snmark}{0}


%% step 2: hanging sidenotes, definition from tugboat
makeatletter
newcommand*{hangfrom}[1]{%
  setbox@tempboxahbox{{#1}}%
  hangindent wd@tempboxa
  noindentbox@tempboxa}
makeatletter

%% step 3: define sidenote marks in the text
newcommand{makesidenotemark}{%
  textsuperscript{thesnmark}
}%


%% step 4: and now the definition of sidenotes
newcommand{sidenote}[1]{%
  refstepcounter{snmark}% Zähler erhöhen
  makesidenotemark{}% Nummer im Text setzen.
  makenote[paragraphs]{%
    hangfrom{%
      makebox[1em][l]{%
        thesnmark}
            }
        protectRaggedRight{} #1}% Text setzen im Rand
}


%% this is, except for the values (».7«) from scrguide.pdf to define
%% the notecolumn.

newlength{paragraphscolwidth}
AfterCalculatingTypearea{%
setlength{paragraphscolwidth}{.4textwidth}%
addtolength{paragraphscolwidth}{-marginparsep}%
}
recalctypearea
DeclareNewNoteColumn[%
position=oddsidemargin+1in
+.7textwidth
+marginparsep,
width=paragraphscolwidth,
font=footnotesize
]{paragraphs}





%% Delete all sidenotes:
%renewcommand{sidenote}[1]{relax}

begin{document}
begin{addmargin}[0pt]{.3textwidth}
  This is the text with a sidenote{See here.blindtext}.
  blindtext


  And here we have a second paragraph and a second
  sidenotesidenote{blindtext}.
  blindtext

  And this is the big hit: We can have sidenotes which include
  pagebreaks!sidenote{Proof of concept: blindtext{}}


  syncwithnotecolumns[paragraphs]

      And now we somewhere have a table:
  begin{table}[b]
    centering
    begin{tabular}{ll}
      Proofsidenote{From inside the float} & of concept\
    end{tabular}
    caption{This is a table}
    label{tab:table}
  end{table}

Besides that, we even can have tabulars in the marginsidenote{%
  Tabular in the margin:

  protectbegin{tabular}{|l|l|}protecthline
    a& b\protecthline
    a& b\protecthline
    a& b\protecthline
  protectend{tabular}
}, but they have
to be narrow and well protected.

end{addmargin}


end{document}

usepackage{environ}
makeatletter
NewEnviron{margintable}{%
  expandafter@margintableexpandafter{BODY}%
}
newcommand*@margintable[1]{%
  makenote[paragraphs]{%
    protectbegin{nonfloattable}detokenize{#1}protectend{nonfloattable}
  }%
}
newenvironment{nonfloattable}{%
  parnoindentbegin{minipage}{linewidth}
    def@captype{table}%
}{%
  end{minipage}par
}

documentclass{scrartcl}
usepackage[T1]{fontenc}
usepackage[utf8]{inputenc}
usepackage{scrlayer-scrpage, scrlayer-notecolumn, lmodern, blindtext,
ragged2e}

%% step 1: counter
newcounter{snmark}
setcounter{snmark}{0}


%% step 2: hanging sidenotes, definition from tugboat
makeatletter
newcommand*{hangfrom}[1]{%
  setbox@tempboxahbox{{#1}}%
  hangindent wd@tempboxa
  noindentbox@tempboxa}
makeatletter

%% step 3: define sidenote marks in the text
newcommand{makesidenotemark}{%
  textsuperscript{thesnmark}
}%


%% step 4: and now the definition of sidenotes
newcommand{sidenote}[1]{%
  refstepcounter{snmark}% Zähler erhöhen
  makesidenotemark{}% Nummer im Text setzen.
  makenote[paragraphs]{%
    hangfrom{%
      makebox[1em][l]{%
        thesnmark}
            }
        protectRaggedRight{} #1}% Text setzen im Rand
}


%% this is, except for the values (».7«) from scrguide.pdf to define
%% the notecolumn.

newlength{paragraphscolwidth}
AfterCalculatingTypearea{%
setlength{paragraphscolwidth}{.4textwidth}%
addtolength{paragraphscolwidth}{-marginparsep}%
}
recalctypearea
DeclareNewNoteColumn[%
position=oddsidemargin+1in
+.7textwidth
+marginparsep,
width=paragraphscolwidth,
font=footnotesize
]{paragraphs}

usepackage{environ}
makeatletter
NewEnviron{margintable}{%
  expandafter@margintableexpandafter{BODY}%
}
newcommand*@margintable[1]{%
  globalletlabeltoset@empty
  @tempcntavalue{table}%
  begin{lrbox}{@tempboxa}
    begin{minipage}{paragraphscolwidth}
      deflabel##1{%
        xdeflabeltoset{%
          labeltoset
          noexpandprotected@writenoexpand@auxout{}{%
            noexpandstringnoexpandnewlabel{##1}{{@currentlabel}{thepage}}}%
        }%
      }%
      def@captype{table}%
      #1%
    end{minipage}
  end{lrbox}%
  setcounter{table}{@tempcnta}%
  makenote[paragraphs]{%
    protectbegin{nonfloattable}detokenize{#1}protectend{nonfloattable}
  }%
  labeltoset
}
newenvironment{nonfloattable}{%
  parnoindentbegin{minipage}{linewidth}
    def@captype{table}%
}{%
  end{minipage}par
}

%% Delete all sidenotes:
%renewcommand{sidenote}[1]{relax}

begin{document}
begin{addmargin}[0pt]{.3textwidth}
  This is the text with a sidenote{See here.blindtext}.
  blindtext


  And here we have a second paragraph and a second
  sidenotesidenote{blindtext}.
  blindtext

  And this is the big hit: We can have sidenotes which include
  pagebreaks!sidenote{Proof of concept: blindtext{}}


  syncwithnotecolumns[paragraphs]

      And now we somewhere have a table:
  begin{table}[b]
    centering
    begin{tabular}{ll}
      Proofsidenote{From inside the float} & of concept\
    end{tabular}
    caption{This is a table}
    label{tab:table}
  end{table}

Besides that, we even can have tabulars in the marginsidenote{%
  Tabular in the margin:

  protectbegin{tabular}{|l|l|}protecthline
    a& b\protecthline
    a& b\protecthline
    a& b\protecthline
  protectend{tabular}
}, but they have
to be narrow and well protected.

begin{margintable}
  centering
  begin{tabular}{|l|l|}hline
    a& b\hline
    a& b\hline
    a& b\hline
  end{tabular}
  caption{This is a second table}%
  label{tab:secondtable}
end{margintable}

See also table~ref{tab:secondtable}.

end{addmargin}

listoftables

end{document}

documentclass{scrartcl}
usepackage[T1]{fontenc}
usepackage[utf8]{inputenc}
usepackage{scrlayer-scrpage, scrlayer-notecolumn, lmodern, blindtext,
ragged2e}

%% step 1: counter
newcounter{snmark}
setcounter{snmark}{0}


%% step 2: hanging sidenotes, definition from tugboat
makeatletter
newcommand*{hangfrom}[1]{%
  setbox@tempboxahbox{{#1}}%
  hangindent wd@tempboxa
  noindentbox@tempboxa}
makeatletter

%% step 3: define sidenote marks in the text
newcommand{makesidenotemark}{%
  textsuperscript{thesnmark}%
}%


%% step 4: and now the definition of sidenotes
newcommand{sidenote}[1]{%
  refstepcounter{snmark}% Zähler erhöhen
  makesidenotemark{}% Nummer im Text setzen.
  makenote[paragraphs]{%
    hangfrom{%
      makebox[1em][l]{%
        thesnmark}%
    }%
    protectRaggedRightdetokenize{#1}}% Text setzen im Rand
}


%% this is, except for the values (».7«) from scrguide.pdf to define
%% the notecolumn.

newlength{paragraphscolwidth}
AfterCalculatingTypearea{%
setlength{paragraphscolwidth}{.4textwidth}%
addtolength{paragraphscolwidth}{-marginparsep}%
}
recalctypearea
DeclareNewNoteColumn[%
position=oddsidemargin+1in
+.7textwidth
+marginparsep,
width=paragraphscolwidth,
font=footnotesize
]{paragraphs}

usepackage{environ}
makeatletter
NewEnviron{margintable}{%
  expandafter@margintableexpandafter{BODY}%
}
newcommand*@margintable[1]{%
  makenote*[paragraphs]{%
    begin{nonfloattable}#1end{nonfloattable}%
  }%
}
newenvironment{nonfloattable}{%
  parnoindentbegin{minipage}{linewidth}
    def@captype{table}%
}{%
  end{minipage}par
}
usepackage{makeidx}

%% Delete all sidenotes:
%renewcommand{sidenote}[1]{relax}

begin{document}
begin{addmargin}[0pt]{.3textwidth}
  This is the text with a sidenotesidenote{See here.blindtext}.
  blindtext


  And here we have a second paragraph and a second
  sidenotesidenote{blindtext}.
  blindtext

  And this is the big hit: We can have sidenotes which include
  pagebreaks!sidenote{Proof of concept: blindtext{}}


  syncwithnotecolumns[paragraphs]

      And now we somewhere have a table:
  begin{table}[b]
    centering
    begin{tabular}{ll}
      Proofsidenote{From inside the float} & of concept\
    end{tabular}
    caption{This is a table}
    label{tab:table}
  end{table}

Besides that, we even can have tabulars in the marginsidenote{%
  Tabular in the margin:

  begin{tabular}{|l|l|}hline
    a& b\hline
    a& b\hline
    a& b\hline
  end{tabular}%
}, but they have
to be narrow and well protected.

begin{margintable}
  centering
  begin{tabular}{|l|l|}hline
    a& b\hline
    a& b\hline
    a& b\hline
  end{tabular}
  caption{This is a second table}%
  label{tab:secondtable}
end{margintable}

See also table~ref{tab:secondtable}.
listoftables

end{addmargin}


end{document}