big doc overhaul

Merged username-removed-24982 requested to merge eighthave/fdroid-website:big-doc-overhaul into master

This merges all of the docs from the manual and wiki into the new Docs section, and updates some of the content in a couple of pages. There aren't any functional changes to how the website works, only documentation changes.

@grote @krt @pserwylo @NicoAlt @uniqx could you take a quick look to make sure I didn't say anything wrong or stupid?