Documentation clean up

This MR cleans up the Gitlab Wiki and moves everything that is not TU specific to the online docs. The online docs will also be cleaned up.

Discussions

• Should we name all main entries index.html even if they are not TOCs?
• Didn't move the Build wiki because it has a lot of outdated and irrelevant info at this point
• We currently have two place for doctest conventions, should we merge those into one?

Changes

The following changes will be applied in this MR:

• Move everything from the gitlab wiki (that is not TU specific) to the online docs
  • Set Up
  • Contributing
  • Testing
  • Documentation
  • Build (not super relevant anymore, also contains a lot of outdated info)
• Clean up the docs/source folder (move every topic to it's own folder)
• Fix the side bar issue (current implementation is to use the read-the-docs theme, from our branch)
• Change every main entry to be index.html, even if it isn't a TOC (i.e. optimize/optimize.rst -> optimize/index.rst). Note that we did not settle on a convention for this yet. It's merely implemented to show how it would look in our current documentation.

Reviewer Notes

Note that the git diff will pretty much be hell. So here are some reviewer notes:

• The moved files support/support.rst, introduction/introduction.rst and installation/installation.rst are unchanged, so they don't need to be reviewed in detail
• Everything that got moved from the Gitlab Wiki should be reviewed in detail, many things changed, especially links and formats, but also some information
• Given that pretty much all changes are inside the online docs, it could be useful to locally build the docs and check the formatting and links
  • Pay special attention to contributing/users-guide-conventions.rst in the section Doctests. This was completely overhauled.