Update markdown requirement from ~=3.2 to ~=3.6

Updates the requirements on [markdown](https://github.com/Python-Markdown/markdown) to permit the latest version.
- [Release notes](https://github.com/Python-Markdown/markdown/releases)
- [Changelog](https://github.com/Python-Markdown/markdown/blob/master/docs/changelog.md)
- [Commits](https://github.com/Python-Markdown/markdown/compare/3.2...3.6)

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

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

docs/requirements.txt

@@ -23,7 +23,7 @@ mkdocs-autolinks-plugin
 
 # Requirements for core
 jinja2~=3.0
-markdown~=3.2
+markdown~=3.6
 mkdocs-material-extensions~=1.3
 pygments~=2.18
 pymdown-extensions~=10.2