fix: docs/requirements.txt to reduce vulnerabilities

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

docs/requirements.txt

@@ -1,4 +1,5 @@
 mkdocs==1.4.3
 sphinx==5.3.0
 sphinx_rtd_theme==1.2.2
-readthedocs-sphinx-search==0.3.1
+readthedocs-sphinx-search==0.3.1
+urllib3>=2.2.2 # not directly required, pinned by Snyk to avoid a vulnerability