#11595 dropped support for python 3.6, but forgot to update this doc.

Co-authored-by: reivilibre <olivier@librepush.net>

Various updates to the install docs.

- Removed page summaries from CONTRIBUTING and installation pages as 
this information was already in the table of contents on the right hand side
- Fixed some broken links in CONTRIBUTING
- Added margin-right tag for when table of contents is being shown
(otherwise the text in the page sometimes overlaps with it)

Our documentation has a history of using a document's name as a way to link to it, such as "See [workers.md]() for details". This makes sense when you're traversing a directory of files, but less sense when the files are abstracted away - as they are on the documentation website.

This PR changes the links to various documentation pages to something that fits better into the surrounding sentence, as you would when making any hyperlink on the web.

Signed-off-by: Dirk Klimpel <dirk@klimpel.org>