docs: document code with doxygen, breathe, and sphinx_csharp

[ReenigneArcher]

Description

Screenshot

Issues Fixed or Closed

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

Branch Updates

LizardByte requires that branches be up-to-date before merging. This means that after any PR is merged, this branch
must be updated before it can be merged. You must also
Allow edits from maintainers.

• I want maintainers to keep my branch updated

[codecov]

Codecov Report

Merging #187 (6c696d6) into master (172e61d) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #187   +/-   ##
=======================================
  Coverage   47.80%   47.80%           
=======================================
  Files           4        4           
  Lines         228      228           
  Branches       16       16           
=======================================
  Hits          109      109           
  Misses        115      115           
  Partials        4        4           

Files	Coverage Δ
Jellyfin.Plugin.Themerr/Api/ThemerrController.cs	0.00% <ø> (ø)