Bump mkdocs-material from 9.2.6 to 9.2.8 in /docs

Bumps [mkdocs-material](https://github.com/squidfunk/mkdocs-material) from 9.2.6 to 9.2.8.
- [Release notes](https://github.com/squidfunk/mkdocs-material/releases)
- [Changelog](https://github.com/squidfunk/mkdocs-material/blob/master/CHANGELOG)
- [Commits](squidfunk/mkdocs-material@9.2.6...9.2.8)

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

Signed-off-by: dependabot[bot] <[email protected]>

docs/requirements.txt

@@ -13,7 +13,7 @@ mike==1.1.2
 mkdocs==1.5.2
 mkdocs-exclude==1.0.2
 mkdocs-macros-plugin==1.0.4
-mkdocs-material==9.2.6
+mkdocs-material==9.2.8
 mkdocs-material-extensions==1.1.1
 packaging==23.1
 Pygments==2.16.1