[headtr1ck]

Description

Several doc pages were moved or removed without a redirect, so their old URLs 404 (e.g. the Tutorials link on xarray.dev). This adds the missing redirects and a CI check so it doesn't happen again:

• Redirects for the five pages moved in Switch documentation to pydata-sphinx-theme #8708 that still had none (tutorials-and-videos, help-diagram, howdoi, ecosystem, getting-started-guide/faq) and for older moved pages (quickstart, why-xray, whats_new, examples, internals, ...)
• New check-doc-redirects job in CI Additional: every page ever added to doc/ must still exist or be in rediraffe_redirects. Pages removed on purpose without a successor are listed in the check.

Checklist

• Closes The "Tutorials" link on http://xarray.dev is broken #11790
• User visible changes (including notable bug fixes) are documented in whats-new.rst

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
    Tools: Claude Code

🤖 Generated with Claude Code

[keewis]

FYI while this will help with the docs (and any old bookmarks or any links we don't control), this won't be a proper fix for the referenced issue: we should modify that link in xarray-contrib/xarray.dev instead

[headtr1ck]

True. Do you think the CI check is overkill?