replace hardcoded urls to docs.pytest.org with internal crossrefs, add crossrefs to numpy docs, use sphinx pep role

[hoefling]

This PR replaces all hardcoded links to https://docs.pytest.org with internal crossreferences (with missing labels being added). Also, hardcoded links to numpy docs were replaced by intersphinx crossrefs. Lastly, all hardcoded references to PEP pages were replaced by usages of Sphinx' :pep: role.

This is the second PR in series, the first one (#9041) replaced hardcoded URLs to stdlib. Also probably #9081 is partially addressed by this.

Apologies for the PR size. This was mostly a semi-automated search-and-replace, with manually fixing broken links afterwards.

[Zac-HD]

I think we should consider leaving the announce changes for later, since they might be used as plain text as well as built with Sphinx.

Otherwise looks good to me 👍

[Zac-HD]

I'm unsure of these ones, since they do render differently.

If we're changing them we should also change the templates https://github.com/pytest-dev/pytest/blob/main/scripts/release.minor.rst and https://github.com/pytest-dev/pytest/blob/main/scripts/release.patch.rst in any case 😄

[hoefling]

@Zac-HD sure thing, reverted the changes in announce dir!

[Zac-HD]

Looks like a solid improvement on the status quo, so let's merge!

Thanks again @hoefling, we really appreciate your work on this - docs and cleanup are often neglected, but are absolutely essential for users 🥰

[Zac-HD]

If you're after a next step, maybe extlinks for issue links (etc)? Here's the setup we use for Hypothesis.

[hoefling]

@Zac-HD yep, this is definitely something I‘d like to tackle on next. Although I haven‘t used extlinks extensively so far, so will look into it first - maybe this extension should also get a checking posttransform similar to what was proposed for intersphinx in Sphinx lately.