Fix broken example and improve some documentation

[tleonhardt]

Fix broken example and improve some documentation

• Fix broken cmd_as_argument.py example whose speak command was crashing because str has no copy method since strings are immutable in Python.
• Update the startup_commands.md documentation to make it more discoverable what the recommended solution is for mixing commands at invocation with a custom argparse parser.
• Update the os.md documentation to make sure it is still accurate

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.64%. Comparing base (fd62071) to head (4bc3674).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1735      +/-   ##
==========================================
- Coverage   99.66%   99.64%   -0.02%     
==========================================
  Files          23       23              
  Lines        5937     5937              
==========================================
- Hits         5917     5916       -1     
- Misses         20       21       +1     

Flag	Coverage Δ
unittests	99.64% <ø> (-0.02%)	⬇️

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