Check newly added links in PR linkcheck runs

[Himanshuagrawal4]

Summary

• Allow source/conf.py to read a linkcheck JSONL baseline and ignore previously reported URLs.
• Restore the cached baseline for PR linkcheck runs so newly added links remain checked.
• Save the latest baseline from successful main-branch linkcheck runs.

Closes #2037

Validation

• git diff --check
• python -m py_compile source/conf.py
• Parsed .github/workflows/test.yml with PyYAML
• Ran a small JSONL parsing smoke test for the baseline ignore logic

Note: full nox/actionlint checks were not run locally because those tools are not installed here.

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📚 Documentation preview 📚: https://python-packaging-user-guide--2067.org.readthedocs.build/en/2067/