fix: doc/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-7267250

doc/requirements.txt

@@ -14,3 +14,4 @@ certifi>=2023.7.22 # not directly required, pinned by Snyk to avoid a vulnerabil
 cryptography>=42.0.2 # not directly required, pinned by Snyk to avoid a vulnerability
 requests>=2.31.0 # not directly required, pinned by Snyk to avoid a vulnerability
 jinja2>=3.1.3 # not directly required, pinned by Snyk to avoid a vulnerability
+urllib3>=2.2.2 # not directly required, pinned by Snyk to avoid a vulnerability