Update sphinx requirement from ~=5.3 to ~=6.1

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [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
...

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

Author: dependabot[bot]

docs/requirements.txt

@@ -1,3 +1,3 @@
-sphinx~=5.3
+sphinx~=6.1
 sphinx-rtd-theme~=1.0
 sphinxcontrib-napoleon~=0.7