Update sphinx requirement from ~=4.1 to ~=5.1

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/5.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v4.1.0...v5.1.1)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

docs/requirements.txt

@@ -1,4 +1,4 @@
-sphinx~=4.1
+sphinx~=5.1
 # >= 1.0.0rc1 for https://github.com/readthedocs/sphinx_rtd_theme/issues/1115
 sphinx-rtd-theme >= 1.0.0rc1
 sphinx-autodoc-typehints