Add PEP 770 to specifications

[sethmlarson]

• Adds reservation of directory names under .dist-info and reserves a few names already in use.
• Adds description of .dist-info/sboms directory.

I wasn't sure whether to add a "guide" or not yet, as there aren't any especially "user-facing" ways to have an SBOM end up in this directory beyond your build backend/build tool supporting it. Should I leave that for a later task, then?

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📚 Documentation preview 📚: https://python-packaging-user-guide--1853.org.readthedocs.build/en/1853/

[webknjaz]

There's probably no need for a guide. Especially if build backends don't have a toggle. It might be useful to note in the existing ones which backend versions implement it.

Also, it might be a good idea to migrate the PEP itself into the specs section.

I've made a few suggestions to RST as I'm trying to keep the style consistent and it's easier to distinguish dirs vs regular files based on the trailing slash..

[webknjaz]

Thanks!