fix: requirements.docs.txt to reduce vulnerabilities

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

Author: snyk-bot

requirements.docs.txt

@@ -8,4 +8,5 @@ mkdocs-literate-nav~=0.6
 mkdocs-section-index~=0.3
 mkdocs-git-authors-plugin~=0.9
 mkdocs-macros-plugin~=1.2
-pymdown-extensions~=10.9
+pymdown-extensions~=10.9
+zipp>=3.19.1 # not directly required, pinned by Snyk to avoid a vulnerability