Bump mkdocs-material from 8.5.5 to 8.5.6 (#1745)

Bumps [mkdocs-material](https://github.com/squidfunk/mkdocs-material) from 8.5.5 to 8.5.6.
- [Release notes](https://github.com/squidfunk/mkdocs-material/releases)
- [Changelog](https://github.com/squidfunk/mkdocs-material/blob/master/CHANGELOG)
- [Commits](https://github.com/squidfunk/mkdocs-material/compare/8.5.5...8.5.6)

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

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requirements-docs.txt

@@ -1,5 +1,5 @@
 mkdocs==1.4.0
-mkdocs-material==8.5.5
+mkdocs-material==8.5.6
 pymdown-extensions==9.6
 mike==1.1.2
 mkdocs-simple-hooks==0.1.5