fix: doc-requirements.txt to reduce vulnerabilities (#416)

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-CERTIFI-5805047

Co-authored-by: snyk-bot <snyk-bot@snyk.io>

doc-requirements.txt

@@ -13,7 +13,7 @@ beautifulsoup4==4.10.0
     #   furo
     #   sphinx-code-include
     #   sphinx-material
-certifi==2021.10.8
+certifi==2023.7.22
     # via requests
 charset-normalizer==2.0.10
     # via requests