Bump mkdocs from 1.2.3 to 1.3.1 (#2171)

Bumps [mkdocs](https://github.com/mkdocs/mkdocs) from 1.2.3 to 1.3.1.
- [Release notes](https://github.com/mkdocs/mkdocs/releases)
- [Commits](mkdocs/mkdocs@1.2.3...1.3.1)

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

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

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

doc/requirements.txt

@@ -1,3 +1,3 @@
-mkdocs==1.2.3
+mkdocs==1.3.1
 python-markdown-math
 mkdocs-material