fix: docs/requirements.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

docs/requirements.txt

@@ -1,3 +1,4 @@
 sphinx-rtd-theme==0.5.2
 sphinxcontrib-contentui==0.2.5
 aiohttp>=3.9.2,<4
+urllib3>=2.5.0 # not directly required, pinned by Snyk to avoid a vulnerability