docs: mention selector chaining

[pranjalbhatia710]

Summary

• explicitly document selector chaining in the Selectors reference
• add a focused faces(">Z").edges(">Y") example showing how each selector narrows the previous selection

Closes #900.

Validation

python3 - <<'PY'
from pathlib import Path
s = Path('doc/selectors.rst').read_text()
assert 'Selector methods can also be chained' in s
assert '.faces(">Z")' in s and '.edges(">Y")' in s
assert '.edges("%Line")' not in s
print('doc/selectors.rst contains explicit selector chaining example')
PY
# doc/selectors.rst contains explicit selector chaining example

git diff --check
# no output

Note: I did not run the full docs build locally because this checkout does not have the OCP/CadQuery runtime dependencies installed (ModuleNotFoundError: No module named 'OCP').

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.67%. Comparing base (a6bedc0) to head (a8a8e53).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2091   +/-   ##
=======================================
  Coverage   95.67%   95.67%           
=======================================
  Files          30       30           
  Lines        9534     9534           
  Branches     1421     1421           
=======================================
  Hits         9122     9122           
  Misses        255      255           
  Partials      157      157           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.