guides, specifications: update for PEP 753

[woodruffw]

This is a first blush at updating the packaging specifications for PEP 753, now that it's been accepted.

Key changes:

1. Home-page and Download-URL have been marked as deprecated and moved into the deprecated section of the metadata spec.
2. well-known-project-urls.rst has been added to reflect the core spec/recommendations in PEP 753.
3. Cross-referenced/linked between the spec above and a bunch of pages that touch on project URL handling.

(2) reflects the core spec part of the PEP, but I've tweaked the language to make it less "PEP-y" based on how the other PyPA living specs seem to be written. But I'm happy to restructure/rewrite it however others please!

Related tasks that I still need to do:

• Update PyPI's metadata handling to match PEP 753 (the current behavior is virtually identical, but there are some edge cases + I'll add appropriate tests)
• Update PyPI's documentation to reference PEP 753
• Other, higher-level guides/docs that should be updated?
• Update the PEP itself to point to the canonical living PyPA spec PEP 753: link to PyPA spec python/peps#4095

CC @pfmoore @ncoghlan @di as interested parties 🙂

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📚 Documentation preview 📚: https://python-packaging-user-guide--1611.org.readthedocs.build/en/1611/

[pfmoore]

A couple of minor comments, but overall LGTM!

[woodruffw]

CI failure looks unrelated (linkcheck seems to be failing due to the current Internet Archive outage).

[willingc]

A few small edits. Thanks @woodruffw.

[woodruffw]

CI appears to have righted itself, so this should be good to go!

[woodruffw]

Gentle ping for review!

[pfmoore]

You have 2 approvals, I think this is ready to go. I can hit the merge button, apparently, but I'm not really an active committer to this repo, so I think it should be one of the guide editors who merges it.

[webknjaz]

Thanks!