Bump mkdocs from 1.4.3 to 1.5.1

Bumps [mkdocs](https://github.com/mkdocs/mkdocs) from 1.4.3 to 1.5.1.
- [Release notes](https://github.com/mkdocs/mkdocs/releases)
- [Commits](mkdocs/mkdocs@1.4.3...1.5.1)

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

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

docs/requirements.txt

@@ -1 +1 @@
-mkdocs==1.4.3
+mkdocs==1.5.1