Bump furo from 2022.6.4.1 to 2022.6.21 in /docs (psf#3138)

Bumps [furo](https://github.com/pradyunsg/furo) from 2022.6.4.1 to 2022.6.21.
- [Release notes](https://github.com/pradyunsg/furo/releases)
- [Changelog](https://github.com/pradyunsg/furo/blob/main/docs/changelog.md)
- [Commits](pradyunsg/furo@2022.06.04.1...2022.06.21)

---
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

@@ -6,4 +6,4 @@ Sphinx==5.0.2
 docutils==0.18.1
 sphinxcontrib-programoutput==0.17
 sphinx_copybutton==0.5.0
-furo==2022.6.4.1
+furo==2022.6.21