Add an XML schema for documentation

This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.

Each class XML's schema conformance is also checked on GitHub Actions.

doc/classes/NavigationMesh.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8" ?>
-<class name="NavigationMesh" inherits="Resource" version="4.0">
+<class name="NavigationMesh" inherits="Resource" version="4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
 	<brief_description>
 		A mesh to approximate the walkable areas and obstacles.
 	</brief_description>