Bump mkdocs from 1.4.2 to 1.5.3 in /requirements

Bumps [mkdocs](https://github.com/mkdocs/mkdocs) from 1.4.2 to 1.5.3.
- [Release notes](https://github.com/mkdocs/mkdocs/releases)
- [Commits](mkdocs/mkdocs@1.4.2...1.5.3)

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

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

requirements/docs.txt

@@ -1,4 +1,4 @@
-mkdocs==1.4.2
+mkdocs==1.5.3
 mkdocs-material==8.5.10
 mkdocs-include-markdown-plugin==4.0.3
 mkdocstrings[python]==0.19.1