Add security as a well-known project URL label

[BHUVANSH855]

This PR adds security (with securitypolicy as a normalisation
alias) to the well-known project URL labels specification.

Live PyPI data shows 573+ projects already using security-related
URL labels, but with 19 different non-normalised strings. This
standardises the most common usage into a single well-known label.

Background research: https://github.com/BHUVANSH855/pypi-security-url-research
Forum discussion: https://discuss.python.org/t/adding-security-as-a-well-known-url-label-and-project-security-table-to-pyproject-toml/108826

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📚 Documentation preview 📚: https://python-packaging-user-guide--2127.org.readthedocs.build/en/2127/

[sethmlarson]

@BHUVANSH855 If this PR lands, would you be willing to submit another PR to Warehouse adding a special icon for this well-known project URL? (maybe fa-shield?)

Here's an example to follow: pypi/warehouse#20466

[BHUVANSH855]

@BHUVANSH855 If this PR lands, would you be willing to submit another PR to Warehouse adding a special icon for this well-known project URL? (maybe fa-shield?)

Here's an example to follow: pypi/warehouse#20466

Yes, i will

[woodruffw]

I could have sworn we disabled the blocking linkcheck here...