Docs: improve/cleanup reference from/to recwarn

[soxofaan]

• Allow maintainers to push and squash when merging my commits. Please uncheck this if you prefer to squash the commits yourself.
• Create a new changelog file in the changelog folder, with a name like <ISSUE NUMBER>.<TYPE>.rst. See changelog/README.rst for details.
• Add yourself to AUTHORS in alphabetical order.

[soxofaan]

I was looking up some docs around the recwarn fixture today and had a hard time getting to the point. This PR tries to fix some problems in the docs around that:

• replace some raw http links with sphinx references
• when mentioning recwarn, point to the API docs instead of a section that barely explains something about recwarn
• from the recwarn API docs, provide cross-reference to a how-to section about recwarn instead of something loosely related
• recwarn API docs: also document __getitem__, __iter__ and __len__ (which are shown in the how to, but not explained)

[nicoddemus]

Thanks!

[patchback]

Backport to 8.3.x: 💚 backport PR created

✅ Backport PR branch: patchback/backports/8.3.x/26a29bdade7efccdf6233942749f02cef57ce694/pr-12866

Backported as #12870

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