fix: docs/requirement.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-TORNADO-6041512

docs/requirement.txt

@@ -8,4 +8,5 @@ ipython>=7.16.3 # not directly required, pinned by Snyk to avoid a vulnerability
 pygments>=2.7.4 # not directly required, pinned by Snyk to avoid a vulnerability
 mistune>=2.0.1 # not directly required, pinned by Snyk to avoid a vulnerability
 setuptools>=65.5.1 # not directly required, pinned by Snyk to avoid a vulnerability
-wheel>=0.38.0 # not directly required, pinned by Snyk to avoid a vulnerability
+wheel>=0.38.0 # not directly required, pinned by Snyk to avoid a vulnerability
+tornado>=6.3.3 # not directly required, pinned by Snyk to avoid a vulnerability