docs: generate the documented command line help from the parsers

[jdhughes-dev]

The usage and option blocks in the README and the docs are the --help output of a command, which was edited by hand and had gone stale: the -fc/-cc option lists were the choices removed when a version suffix was allowed, make-program listed zonbud3 rather than zonbud, and getting_started used the argparse heading from before python 3.10.

The blocks are now written by scripts/update_docs.py (pixi run update-docs), and test_docs_current asserts they match the parsers. The parser for each command is split out of the function that runs it so the help can be rendered without parsing arguments.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.264%. Comparing base (5f1fdc5) to head (a71ecf6).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
pymake/cmds/build.py	71.428%	2 Missing ⚠️
pymake/cmds/mfpymakecli.py	75.000%	1 Missing ⚠️
pymake/pymake_parser.py	80.000%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           develop      #316       +/-   ##
=============================================
+ Coverage   79.671%   81.264%   +1.593%     
=============================================
  Files           20        20               
  Lines         3591      3779      +188     
=============================================
+ Hits          2861      3071      +210     
+ Misses         730       708       -22     

Files with missing lines	Coverage Δ
pymake/cmds/mfpymakecli.py	64.705% <75.000%> (+64.705%)	⬆️
pymake/pymake_parser.py	81.250% <80.000%> (+45.766%)	⬆️
pymake/cmds/build.py	53.061% <71.428%> (+53.061%)	⬆️

... and 3 files with indirect coverage changes

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