fix: doc-requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-6002459

Author: snyk-bot

doc-requirements.txt

@@ -96,7 +96,7 @@ text-unidecode==1.3
     # via python-slugify
 unidecode==1.3.2
     # via python-slugify
-urllib3==1.26.7
+urllib3==1.26.18
     # via requests
 
 # The following packages are considered to be unsafe in a requirements file: