[VeckoTheGecko]

Description

Adopts sphinxext-rediraffe so that docs pages can be moved or deleted without breaking existing links.

• Adds sphinxext-rediraffe to the docs Pixi environment and enables it in docs/conf.py.
• Adds an empty rediraffe_redirects = {} dict to docs/conf.py. Entries map old paths to new paths, relative to docs/, e.g. "user_guide/examples/tutorial_old.ipynb": "user_guide/examples/tutorial_new.ipynb". It's empty for now because no pages have been moved or deleted since v4.0.0. URLs from the v3 docs aren't redirected, since they still work under their own Read the Docs version.
• Adds a sentence to the development guide asking contributors to add an entry when they move or delete a page.

During the HTML build, rediraffe writes a small redirect page for each entry, e.g. old.html that forwards to new.html and keeps the query string and anchor. Chained redirects are resolved, so users are redirected at most once.

This follows CPython's setup, which also keeps a dict in conf.py and relies on review to catch missing redirects. rediraffe also has builders that check git for renamed or deleted pages without a redirect, but I left those out to keep this simple. Pages rarely move, and a reviewer can spot renames and deletions in the PR's file list.

Testing: I built the HTML docs (without running the notebooks) with:

1. the empty dict: the build succeeds and rediraffe gives no warnings.
2. a temporary entry "old_installation.md": "user_guide/getting_started/installation.md" (not included in this PR): the build creates old_installation.html, which redirects to the installation page.

Checklist

• xref [DOCS] Changing user guide to a nested structure #3038 (review)
• This PR targets the correct branch (main for normal development, v3-support for v3 support)

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.
  • Describe how you used it (e.g., by pasting your prompt): Claude Code (Opus 5.5) was asked to "branch off of main that adopts https://sphinxext-rediraffe.readthedocs.io/". It made the changes and ran the testing described above. I chose the scope: no v3 backfill, and a minimal CPython-style setup without the redirect-checking builders.

🤖 Generated with Claude Code

[VeckoTheGecko]

merging, will test in the other PR