Bump mkdocs-material from 9.1.21 to 9.2.6 in /docs

Bumps [mkdocs-material](https://github.com/squidfunk/mkdocs-material) from 9.1.21 to 9.2.6.
- [Release notes](https://github.com/squidfunk/mkdocs-material/releases)
- [Changelog](https://github.com/squidfunk/mkdocs-material/blob/master/CHANGELOG)
- [Commits](squidfunk/mkdocs-material@9.1.21...9.2.6)

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

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

docs/requirements.txt

@@ -1,5 +1,5 @@
 mkdocs==1.5.2
-mkdocs-material==9.1.21
+mkdocs-material==9.2.6
 mkdocs-include-markdown-plugin==6.0.1
 mkdocstrings[python]==0.22.0