Bump mkdocs from 1.3.1 to 1.4.0 (#2250)

Bumps [mkdocs](https://github.com/mkdocs/mkdocs) from 1.3.1 to 1.4.0.
- [Release notes](https://github.com/mkdocs/mkdocs/releases)
- [Commits](mkdocs/mkdocs@1.3.1...1.4.0)

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

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

doc/requirements.txt

@@ -1,3 +1,3 @@
-mkdocs==1.3.1
+mkdocs==1.4.0
 python-markdown-math
 mkdocs-material