Bump sphinx from 5.3.0 to 6.1.3 in /docs

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 5.3.0 to 6.1.3.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES)
- [Commits](sphinx-doc/sphinx@v5.3.0...v6.1.3)

---
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==1.0.0
-Sphinx==5.3.0
+Sphinx==6.1.3
 # Older versions break Sphinx even though they're declared to be supported.
 docutils==0.19
 sphinxcontrib-programoutput==0.17