fix: requirements-docs.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-MARKDOWN-15428352

Author: snyk-bot

requirements-docs.txt

@@ -1,2 +1,3 @@
 mkdocs
-mkdocs-jupyter
+mkdocs-jupyter
+markdown>=3.8.1 # not directly required, pinned by Snyk to avoid a vulnerability