Add PyPI extlink alias

[Herrtian]

Adds the requested PyPI extlinks alias, so docs can use the :pypi: role for links to project pages.

I kept this to the alias itself and did not do any broad link rewrite in the same change.

Checks:

• git diff --check
• uv run --with pre-commit pre-commit run --files source/conf.py
• uv run --with nox nox -s build

Fixes #1370

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📚 Documentation preview 📚: https://python-packaging-user-guide--2041.org.readthedocs.build/en/2041/