refactor: Merge user, admin and developer documentation

[MoritzWeber0]

Previously, we had four different types of documentation:

• Developer documentation (exposed via Github pages)
• User documentation (exposed via individual instances)
• Admin documentation (partially in user docs, partially in README/CONRIBUTING)
• Getting started (in Github repository)

The number of different documentations led to increased maintainance effort. Also, there was no single source of truth.

In this commit, all four documentation types are merged into one mkdocs documentation. The documentation lived in the docs directory. Switching between the different documentation types is now done via navigation tabs. The new documentation is exposed via Github pages, making the user documentation also available when no instance is running.

In addition to the merge, there were some other small changes:

• A new Makefile in the docs directory is added.
• The documentation is now part of the dev target in the Makefile. Run it via make -j4 dev.
• The code style development documentation was splitted into different backend and frontend code style pages.
• Most of the information in the CONTRIBUTING.md is moved to the development documentation.
• The development setup guide was simplified and and the focus is on getting everything running at the same time, not each service individually. All optional configuration options are moved to other parts of the documentation.
• The uninstall guide of the README.md is moved to the admin docs.
• A custom style.css is added to always show a scroll bar. It prevents vertical jumping when the scroll bar appears.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3feca10) 71.64% compared to head (0795abc) 71.64%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1107   +/-   ##
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  Coverage   71.64%   71.64%           
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  Files         160      160           
  Lines        5078     5078           
  Branches      544      544           
=======================================
  Hits         3638     3638           
  Misses       1336     1336           
  Partials      104      104           

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[amolenaar]

Looks good. I found a few improvements.

Also updated the Makefile, since the docs container could no longer be built.