fix: doc/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-FONTTOOLS-15869939

Author: snyk-bot

doc/requirements.txt

@@ -12,4 +12,5 @@ matplotlib
 seaborn
 mypy_extensions>=0.4
 # mistune<2
-jupyterlab>=4.4.8 # not directly required, pinned by Snyk to avoid a vulnerability
+jupyterlab>=4.4.8 # not directly required, pinned by Snyk to avoid a vulnerability
+fonttools>=4.62.0 # not directly required, pinned by Snyk to avoid a vulnerability