docs: remove some excessive nesting of sections in the User Guide

The main goal is to eliminate the seventh level headings which are not
even possible to render in HTML. But this hopefully makes the structure
a bit more sensible and produces a saner ToC as well.

Change-Id: I31fd100b5129b2fcbcb3d3a346ac698fc75ece5e
Merge "docs: remove some excessive nesting of sections in the User Guide" into develop