fix: docs/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-PYMDOWNEXTENSIONS-12427119

Author: snyk-bot

docs/requirements.txt

@@ -1,4 +1,5 @@
 mkdocs==1.1.2
 mkdocs-material==7.1.7
 markdown_include==0.6.0
-pygments==2.7.4
+pygments==2.7.4
+pymdown-extensions>=10.16.1 # not directly required, pinned by Snyk to avoid a vulnerability