docs: document Fixed sketch constraint

[pranjalbhatia710]

Summary

• add the documented Fixed sketch constraint to the sketch constraints table
• align the table with the example just above it and the implemented sketch solver constraint kinds

Closes #2077.

Validation

git diff --check
python3 - <<'PY'
from pathlib import Path
text = Path('cadquery/occ_impl/sketch_solver.py').read_text()
doc = Path('doc/sketch.rst').read_text()
assert '"Fixed": (1, ("CIRCLE", "LINE"), NoneType, None)' in text
assert '* - Fixed\n      - 1\n      - All\n      - None\n      - Entire entity is fixed in place' in doc
print('validated Fixed sketch constraint source entry and docs table row')
PY
# validated Fixed sketch constraint source entry and docs table row

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.72%. Comparing base (95a4744) to head (18c2339).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2086   +/-   ##
=======================================
  Coverage   95.72%   95.72%           
=======================================
  Files          30       30           
  Lines        9533     9533           
  Branches     1421     1421           
=======================================
  Hits         9125     9125           
  Misses        253      253           
  Partials      155      155           

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