Add documentation to repository

[pixeldesu]

So the setup is known to people outside of GitHub and the documentation isn't lost if the Wiki feature disappears or whatever happens in the future.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.16%. Comparing base (1b86fb3) to head (2eef0e0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2185   +/-   ##
=======================================
  Coverage   96.16%   96.16%           
=======================================
  Files         181      181           
  Lines        2844     2844           
=======================================
  Hits         2735     2735           
  Misses        109      109           

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