additional info:
mainpage1 and mainpage2 have no content for themselves (just redirection pages), they are just a way of grouping the pages in the backend

default behaviour:
- mainpage1 --> /mainpage1
- - subpage1_1 --> /mainpage1/subpage1_1
- - subpage1_2 --> /mainpage1/subpage1_2
- mainpage2 --> /mainpage2
- - subpage2_1 --> /mainpage2/subpage2_1
- - subpage2_2 --> /mainpage2/subpage2_2

desired behaviour:
- mainpage1 --> /mainpage1
- - subpage1_1 --> /subpage1_1
- - subpage1_2 --> /subpage1_2
- mainpage2 --> /mainpage2
- - subpage2_1 --> /subpage2_1
- - subpage2_2 --> /subpage2_2