fix: docs/requirements.txt to reduce vulnerabilities

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

Author: snyk-bot

docs/requirements.txt

@@ -1,2 +1,3 @@
 mkdocs==1.3.0
-mkdocs-render-swagger-plugin==0.0.3
+mkdocs-render-swagger-plugin==0.0.3
+zipp>=3.19.1 # not directly required, pinned by Snyk to avoid a vulnerability