fix: requirements/requirements_docs.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-10390193
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-10390194

Author: snyk-bot

requirements/requirements_docs.txt

@@ -1,4 +1,5 @@
 sphinx>=4.0.1
 #m2r2
 sphinxcontrib-programoutput
-#mistune==0.8.4
+#mistune==0.8.4
+urllib3>=2.5.0 # not directly required, pinned by Snyk to avoid a vulnerability