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/EditorImportPlugin.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8" ?>
-<class name="EditorImportPlugin" inherits="ResourceImporter" version="4.0">
+<class name="EditorImportPlugin" inherits="ResourceImporter" version="4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
 	<brief_description>
 		Registers a custom resource importer in the editor. Use the class to parse any file and import it as a new resource type.
 	</brief_description>