docs: explain selector chaining

[pranjalbhatia710]

Summary

• add an explicit explanation of selector chaining to the selectors reference
• include a focused example that narrows from faces to circular edges before chamfering

Fixes #900

Test plan

• python3 - <<'PY' ... sanity check that the selector chaining section exists and cadquery directive bodies are indented
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.64%. Comparing base (9633409) to head (dfdc217).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2042   +/-   ##
=======================================
  Coverage   95.64%   95.64%           
=======================================
  Files          30       30           
  Lines        8962     8962           
  Branches     1328     1328           
=======================================
  Hits         8572     8572           
  Misses        240      240           
  Partials      150      150           

☔ View full report in Codecov by Sentry.
📢 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.