fix: docs/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-JINJA2-6150717
- https://snyk.io/vuln/SNYK-PYTHON-JINJA2-6809379

docs/requirements.txt

@@ -2,4 +2,5 @@ mkdocs==1.3.0
 mkdocs-material==7.3.3
 mkdocs-macros-plugin==0.6.0
 mkdocs-redirects==1.0.3
-pygments>=2.7.4 # not directly required, pinned by Snyk to avoid a vulnerability
+pygments>=2.7.4 # not directly required, pinned by Snyk to avoid a vulnerability
+jinja2>=3.1.4 # not directly required, pinned by Snyk to avoid a vulnerability