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

@@ -6,5 +6,5 @@ requests>=2.32.0 # not directly required, pinned by Snyk to avoid a vulnerabilit
 setuptools>=70.0.0
 sphinx==5.3.0
 sphinx-gallery==0.8.1
-urllib3>=2.2.2 # not directly required, pinned by Snyk to avoid a vulnerability
+urllib3>=2.6.3 # not directly required, pinned by Snyk to avoid a vulnerability
 zipp>=3.19.1 # not directly required, pinned by Snyk to avoid a vulnerability