docs: Improve navigation of docs by making headers more consistent
Created by: mrnugget
This does multiple things:
- Rename some headers to result in a more uniform naming of headers
- Fix styling of sub-headers
- Reorder headers to match the navigational-structure that's shown on index pages
- Get rid of duplicate content
I have a few more improvements for the docs, but I think shipping small PRs makes more sense, since this is hard to review.