Improve TypedDict documentation

[AlexWaygood]

• Our source code had several confusing comments referring to PEP 728 not yet having been implemented upstream. It's been implemented upstream for a while now, but we can't yet simply re-export TypedDict on Python 3.15 because...
• ...We never documented that typing_extensions includes experimental support for PEP 764 (inline TypedDicts).

This PR improves the documentation for TypedDicts. To test the PR, I built the docs locally, which exposed:

• I didn't know how to do that for this repo. This PR adds some notes to CONTRIBUTING.md on how to do that.
• The docs build already failed on main with nitpicky mode, because the signature we gave for dataclass_transform in the docs was invalid (you can't do multiline definitions like that with Sphinx). This PR therefore also fixes those issues, and adds a CI job to build the docs with nitpicky mode, to prevent issues like that from reoccurring.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.26%. Comparing base (224f8d5) to head (76663c0).

@@            Coverage Diff             @@
##             main     #770      +/-   ##
==========================================
- Coverage   97.26%   97.26%   -0.01%     
==========================================
  Files           3        3              
  Lines        7862     7860       -2     
==========================================
- Hits         7647     7645       -2     
  Misses        215      215              

Flag	Coverage Δ
3.10	88.85% <100.00%> (-0.01%)	⬇️
3.10.4	88.85% <100.00%> (-0.01%)	⬇️
3.11	88.02% <100.00%> (-0.01%)	⬇️
3.11.0	87.27% <100.00%> (-0.01%)	⬇️
3.12	87.97% <100.00%> (-0.01%)	⬇️
3.12.0	87.96% <100.00%> (-0.01%)	⬇️
3.13	83.39% <100.00%> (-0.01%)	⬇️
3.13.0	84.10% <100.00%> (-0.01%)	⬇️
3.14	79.53% <100.00%> (-0.01%)	⬇️
3.9	89.53% <100.00%> (-0.01%)	⬇️
3.9.12	89.53% <100.00%> (-0.01%)	⬇️
pypy3.10	88.68% <100.00%> (-0.01%)	⬇️
pypy3.11	87.88% <100.00%> (-0.01%)	⬇️
pypy3.9	89.37% <100.00%> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
src/test_typing_extensions.py	98.23% <100.00%> (-0.01%)	⬇️
src/typing_extensions.py	94.00% <100.00%> (ø)

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