Update mkdocs-material requirement from ~=9.1 to ~=9.4

Updates the requirements on [mkdocs-material](https://github.com/squidfunk/mkdocs-material) to permit the latest version.
- [Release notes](https://github.com/squidfunk/mkdocs-material/releases)
- [Changelog](https://github.com/squidfunk/mkdocs-material/blob/master/CHANGELOG)
- [Commits](squidfunk/mkdocs-material@9.1.0...9.4.0)

---
updated-dependencies:
- dependency-name: mkdocs-material
  dependency-type: direct:production
...

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

requirements_docs.txt

@@ -2,5 +2,5 @@ invoke~=2.2
 mike~=1.1
 mkdocs~=1.5
 mkdocs-awesome-pages-plugin~=2.9
-mkdocs-material~=9.1
+mkdocs-material~=9.4
 mkdocstrings[python]~=0.22.0