Bump sphinx from 8.1.3 to 8.2.1 in /docs (#4587)

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 8.1.3 to 8.2.1.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/v8.2.1/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v8.1.3...v8.2.1)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

docs/requirements.txt

@@ -1,7 +1,7 @@
 # Used by ReadTheDocs; pinned requirements for stability.
 
 myst-parser==4.0.1
-Sphinx==8.1.3
+Sphinx==8.2.1
 # Older versions break Sphinx even though they're declared to be supported.
 docutils==0.21.2
 sphinxcontrib-programoutput==0.18