use the docs sidebar to be the TOC means that level 2 headings are the most important

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
Merge pull request #5074 from SvenDowideit/simplify-builder-headings

use the docs sidebar to be the TOC means that level 2 headings are the most important