Docs: fix security issue with generator version

.readthedocs.yml

@@ -8,3 +8,8 @@ version: 2
 # Build documentation with MkDocs
 mkdocs:
   configuration: mkdocs.yml
+
+# Declare the Python requirements required to build your docs
+python:
+   install:
+   - requirements: docs/requirements.txt

docs/requirements.txt

@@ -1,2 +1,2 @@
-Markdown==3.2.2
-mkdocs==1.1.2
+Markdown==3.3.4
+mkdocs==1.2.3