fix: doc/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-REQUESTS-6928867
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-7267250
- https://snyk.io/vuln/SNYK-PYTHON-ZIPP-7430899

doc/requirements.txt

@@ -5,4 +5,6 @@ sphinxcontrib-spelling==8.0.0
 certifi>=2023.7.22 # not directly required, pinned by Snyk to avoid a vulnerability
 jinja2>=3.1.3 # not directly required, pinned by Snyk to avoid a vulnerability
 pygments>=2.15.0 # not directly required, pinned by Snyk to avoid a vulnerability
-requests>=2.32.0 # not directly required, pinned by Snyk to avoid a vulnerability
+requests>=2.32.2 # not directly required, pinned by Snyk to avoid a vulnerability
+urllib3>=2.2.2 # not directly required, pinned by Snyk to avoid a vulnerability
+zipp>=3.19.1 # not directly required, pinned by Snyk to avoid a vulnerability