Bump mkdocs from 1.1.2 to 1.2.3 in /docs

Bumps [mkdocs](https://github.com/mkdocs/mkdocs) from 1.1.2 to 1.2.3.
- [Release notes](https://github.com/mkdocs/mkdocs/releases)
- [Commits](https://github.com/mkdocs/mkdocs/compare/1.1.2...1.2.3)

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

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

docs/requirements.txt

@@ -22,7 +22,7 @@ lxml==4.6.3
 markdown==3.2.2
 markupsafe==1.1.1
 mkautodoc==0.1.0
-mkdocs==1.1.2
+mkdocs==1.2.3
 mock==4.0.1
 more-itertools==8.2.0
 mypy==0.761