Bump sphinx from 5.0.2 to 7.2.6 in /docs

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 5.0.2 to 7.2.6.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v5.0.2...v7.2.6)

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

Signed-off-by: dependabot[bot] <support@github.com>

docs/requirements.txt

@@ -1,7 +1,7 @@
 # Used by ReadTheDocs; pinned requirements for stability.
 
 myst-parser==0.18.0
-Sphinx==5.0.2
+Sphinx==7.2.6
 # Older versions break Sphinx even though they're declared to be supported.
 docutils==0.18.1
 sphinxcontrib-programoutput==0.17