docs: add documentation for developers

[phm07]

Add basic documentation for developers. This should make it easier to contribute. Also it is in preparation for subcommand groups, where we need documentation on which groups subcommands belong to.

https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c5e3f00) 45.51% compared to head (12cba30) 45.46%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #682      +/-   ##
==========================================
- Coverage   45.51%   45.46%   -0.06%     
==========================================
  Files         179      179              
  Lines        7879     7890      +11     
==========================================
+ Hits         3586     3587       +1     
- Misses       3837     3846       +9     
- Partials      456      457       +1     

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