docs: Initial user facing docs

[jannfis]

What does this PR do / why we need it:

This PR introduces initial user facing docs, ready to be rendered by readthedocs.org.

It's by far not complete or really valuable to users yet, but it allows us to setup the RTD instance, GitHub actions rendering and is ready for further contributions.

Which issue(s) this PR fixes:

Fixes #?

How to test changes / Special notes to the reviewer:

You can checkout the PR's code and run make serve-docs to start a local RTD container to view the documentation.

Checklist

• Documentation update is required by this PR (and has been updated) OR no documentation update is required.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 48.32%. Comparing base (d4f380d) to head (d82e61e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #276      +/-   ##
==========================================
+ Coverage   48.20%   48.32%   +0.11%     
==========================================
  Files          55       55              
  Lines        5066     5066              
==========================================
+ Hits         2442     2448       +6     
+ Misses       2424     2417       -7     
- Partials      200      201       +1     

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