docs: explain Workplane tag and end usage

[pranjalbhatia710]

Closes #666.

This adds a small Workplane docs section for the two stack helpers called out in the issue:

• explains how end() returns to the parent Workplane after a selection
• explains how tag() stores a stable Workplane reference in the modelling context
• shows a short example that tags mating-style faces, returns to the base part, and later selects from the tagged Workplanes with the selector tag argument

Validation:

• python3 AST parse of the added code snippets
• git diff --check

I could not run the full Sphinx docs build locally because the checkout does not currently have CadQuery's runtime/doc dependencies installed (import cadquery fails on missing runtype).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.64%. Comparing base (9633409) to head (0b86d4f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2044   +/-   ##
=======================================
  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 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.