fix: docs/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-MKDOCSMATERIAL-7856160

docs/requirements.txt

@@ -1,5 +1,5 @@
 mkdocs==1.3.0
-mkdocs-material==7.3.3
+mkdocs-material==9.5.32
 mkdocs-macros-plugin==0.6.0
 mkdocs-redirects==1.0.3
 pygments>=2.7.4 # not directly required, pinned by Snyk to avoid a vulnerability