Add initial guide for maintaining Software Bill-of-Materials

[sethmlarson]

Part of python/cpython#112302
Closes #1241

This adds an initial guide for maintaining CPython's SBOM document for core developers and adds a future location for the docs for later additions to CPython's SBOM tooling (e.g. when I implement external/optional dependencies from cpython-source-deps).

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📚 Documentation preview 📚: https://cpython-devguide--1251.org.readthedocs.build/

[hugovk]

Thanks for adding this!

[sethmlarson]

Thanks for the review @hugovk, I believe I've incorporated all of your suggestions :)

[sethmlarson]

Thanks for the review @hugovk, I've done the following:

• Flattened the embedded lists as you recommended.
• Switched the numbered list of items back to unnumbered
• Split the "Remove" section to be separate
• Added a lot more detail into describing which SPDX SBOM fields to use for information.

[hugovk]

Thanks, looks good! Just some little nits!

(You can click "Add suggestion to batch" and then commit them all at the same time.)

[sethmlarson]

@hugovk Done! Thanks for teaching me about that GitHub feature btw, I hadn't used it before since it always seemed to be greyed out :)

[hugovk]

Thank you!

We can link to this page on the CI when make regen-sbom fails.

[sethmlarson]

I'm happy to merge this whenever, I'll take it as an action to link to the new page once available in the regen-sbom CI job too.

[hugovk]

Let's merge now, we can make adjustments whenever we need to. Thanks again!