Document macOS development setup

[kaashif]

Summary

• document GAP installation through Homebrew on macOS
• explain how to make a development checkout override the bundled package
• add commands to verify the loaded package path and run documentation tests and the test suite
• clarify that LaTeX is only needed to generate the PDF manual

Verification

• GAP 4.15.1 on Apple Silicon macOS
• GAPDoc, GRAPE, and IO load successfully
• gap -q < makedoc.g succeeds
• gap -q tst/testall.g reports 0 failures in 5 files

[kaashif]

looks good

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.39%. Comparing base (49bc63b) to head (1712809).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #28   +/-   ##
=======================================
  Coverage   54.39%   54.39%           
=======================================
  Files          25       25           
  Lines        1355     1355           
=======================================
  Hits          737      737           
  Misses        618      618           

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