fix: doc/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-JUPYTERLAB-13053585

Author: snyk-bot

doc/requirements.txt

@@ -11,4 +11,5 @@ nbformat
 matplotlib
 seaborn
 mypy_extensions>=0.4
-# mistune<2
+# mistune<2
+jupyterlab>=4.4.8 # not directly required, pinned by Snyk to avoid a vulnerability