Bump sphinx from 4.1.2 to 4.2.0 in /requirements

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 4.1.2 to 4.2.0.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/4.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v4.1.2...v4.2.0)

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

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

Author: dependabot[bot]

requirements/docs.txt

@@ -1 +1 @@
-Sphinx==4.1.2
+Sphinx==4.2.0