[DOC] Improve tree view colors attribute description

Add example for tree view colors attribute. Add note about escaping < and >. 
Add "(Tree views)" to the header additionally, as in other places in documentation, <tree> is still called  "Tree view"
Add link to the reference of the tree view from the backend tutorial
Should use a `note` or `warning` directive directive. Also that's a basic property of XML documents, not sure that belongs.