Build(deps): Bump furo from 2021.9.8 to 2021.9.22 in /docs (#474)

Bumps [furo](https://github.com/pradyunsg/furo) from 2021.9.8 to 2021.9.22.
- [Release notes](https://github.com/pradyunsg/furo/releases)
- [Changelog](https://github.com/pradyunsg/furo/blob/main/docs/changelog.md)
- [Commits](pradyunsg/furo@2021.09.08...2021.09.22)

---
updated-dependencies:
- dependency-name: furo
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

docs/requirements.txt

@@ -1,2 +1,2 @@
 sphinx==4.2.0
-furo==2021.9.8
+furo==2021.9.22