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

@@ -1,6 +1,6 @@
 mkdocs==1.4.2
 mkdocs-material==9.0.5
-pymdown-extensions>=9.9.1
+pymdown-extensions>=10.16.1
 mkdocs_pymdownx_material_extras==2.3.1
 mkdocs-minify-plugin==0.6.2
 mike==1.1.2