Bump sphinx from 8.1.3 to 8.2.1 in /docs

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]>

Author: dependabot[bot]

docs/requirements.txt

@@ -1,4 +1,4 @@
-sphinx==8.1.3
+sphinx==8.2.1
 sphinx-book-theme>=1.0.1
 sphinx-click==6.0.0
 myst_parser==4.0.0