DOC/BF: drop `rich_argparse` for home-grown solution

[aqw]

This has multiple advantages:

• drops a dependency (that is also unpackaged in Debian)
• rst_to_rich() is only used in the parser, thus rich markup no longer leaks into sphinx output
• fixes the "usage" section of sphinx output (which was garbled with CLI colorization syntax)

I also moved the commit from #616 to cleanup main.py that I missed in #615 that @TobiasKadelka pointed out.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.87179% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 95.83%. Comparing base (139a8f5) to head (c838d10).

Files	Patch %	Lines
onyo/main.py	94.11%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #617 +/- ## ========================================== - Coverage 95.85% 95.83% -0.02% ========================================== Files 67 67 Lines 5640 5647 +7 Branches 1103 1105 +2 ========================================== + Hits 5406 5412 +6 Misses 132 132 - Partials 102 103 +1 ```

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[TobiasKadelka]

This looks very cool!