docs: Use PyData Sphinx Theme version switcher

[kratsg]

Pull Request Description

Use the PyData Sphinx Theme version switcher and version warning banner instead of the custom hand-rolled solution currently in the docs.

Closes #2516.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

---

* Add `docs/_static/switcher.json` with all stable releases mapping versions
  to their ReadTheDocs URLs.
* Configure `html_theme_options` in `docs/conf.py` to enable the PyData
  Sphinx Theme version switcher and version warning banner, using
  `READTHEDOCS_VERSION` to set `version_match` at build time.
* Remove the hand-rolled dev-version warning banner (raw HTML in
  `index.rst` and `citations.rst`, JS detection in `custom.js`, and
  CSS in `custom.css`) now that `show_version_warning_banner` from the
  PyData Sphinx Theme covers the same use case.
* Exempt `docs/_static/switcher.json` from the `docs/_*/` gitignore rule.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.28%. Comparing base (24b1013) to head (e631adc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2694   +/-   ##
=======================================
  Coverage   98.28%   98.28%           
=======================================
  Files          65       65           
  Lines        4305     4305           
  Branches      465      465           
=======================================
  Hits         4231     4231           
  Misses         46       46           
  Partials       28       28           

Flag	Coverage Δ
contrib	98.16% <ø> (ø)
doctest	98.28% <ø> (ø)
unittests-3.10	96.46% <ø> (ø)
unittests-3.11	96.46% <ø> (ø)
unittests-3.12	96.46% <ø> (ø)
unittests-3.13	96.46% <ø> (ø)
unittests-3.9	96.53% <ø> (ø)

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