Refactor (docs, www): make a GraphqlEmbed MDX

<iframe title="A basic query" src="https

<GraphqlEmbed title="Using limit" query={`
{
  allMarkdownRemark(limit: 3) {
    totalCount
    edges {
      node {
        frontmatter {
          title
        }
      }
    }
  }
}
`}/>
get link:https://ouo.io/kT46ZA