Bump sphinx from 4.0.2 to 6.0.0 in /docs

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 4.0.2 to 6.0.0.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES)
- [Commits](sphinx-doc/sphinx@v4.0.2...v6.0.0)

---
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

@@ -1,2 +1,2 @@
-sphinx==4.0.2
+sphinx==6.0.0
 insegel==1.1.0