chore(deps): bump mkdocs from 1.1.2 to 1.6.0

Bumps [mkdocs](https://github.com/mkdocs/mkdocs) from 1.1.2 to 1.6.0.
- [Release notes](https://github.com/mkdocs/mkdocs/releases)
- [Commits](mkdocs/mkdocs@1.1.2...1.6.0)

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

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

requirements.docs.txt

@@ -1 +1 @@
-mkdocs==1.1.2
+mkdocs==1.6.0