Bump markdown from 3.3.4 to 3.3.6

Bumps [markdown](https://github.com/Python-Markdown/markdown) from 3.3.4 to 3.3.6.
- [Release notes](https://github.com/Python-Markdown/markdown/releases)
- [Commits](https://github.com/Python-Markdown/markdown/compare/3.3.4...3.3.6)

---
updated-dependencies:
- dependency-name: markdown
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

docs/requirements.txt

@@ -1,6 +1,6 @@
 git+https://github.com/fralau/mkdocs_macros_plugin@v0.5.12#egg=mkdocs-macros-plugin
 git+https://github.com/g-provost/lightgallery-markdown@v0.5#egg=lightgallery
-markdown==3.3.4
+markdown==3.3.6
 mkdocs-awesome-pages-plugin==2.6.0
 mkdocs-git-revision-date-localized-plugin==0.10.2
 mkdocs-material-extensions==1.0.3