Check new links against linkcheck baseline

[Herrtian]

Fixes #2037.

This caches the linkcheck JSON output from main branch runs and uses it as a baseline on pull requests, so known URLs can be ignored while newly added links still get checked.

Checks:

• Parsed workflow YAML with PyYAML
• Exercised the baseline loader with sample JSONL
• uv run --with pre-commit pre-commit run --files .github/workflows/test.yml source/conf.py
• uv run --with nox nox -s build
• py -m py_compile source\conf.py
• git diff --check

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📚 Documentation preview 📚: https://python-packaging-user-guide--2056.org.readthedocs.build/en/2056/