fix: docs/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-14896210

Author: snyk-bot

docs/requirements.txt

@@ -1,4 +1,5 @@
 Sphinx~=5.0.1
 sphinx_autodoc_typehints~=1.18.3
 docutils<0.18
-sphinx-rtd-theme~=1.0.0
+sphinx-rtd-theme~=1.0.0
+urllib3>=2.6.3 # not directly required, pinned by Snyk to avoid a vulnerability