Bump sphinx from 4.1.1 to 4.2.0 in /docs

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 4.1.1 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@v4.1.1...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>

docs/requirements.txt

@@ -1,4 +1,4 @@
-sphinx==4.1.1
+sphinx==4.2.0
 sphinx-click==3.0.1
 sphinx-rtd-theme==0.5.2
 click==8.0.1