Bump mkdocs-material from 4.1.2 to 4.2.0 (#4923)

Bumps [mkdocs-material](https://github.com/squidfunk/mkdocs-material) from 4.1.2 to 4.2.0.
- [Release notes](https://github.com/squidfunk/mkdocs-material/releases)
- [Changelog](https://github.com/squidfunk/mkdocs-material/blob/master/CHANGELOG)
- [Commits](https://github.com/squidfunk/mkdocs-material/compare/4.1.2...4.2.0)

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

requirements/docs.txt

@@ -3,5 +3,5 @@
 # This file is used by Read the Docs, Travis and when building locally.
 
 mkdocs==1.0.4
-mkdocs-material==4.1.2
+mkdocs-material==4.2.0
 mdx_truly_sane_lists==1.2