[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
agree, this could be just a reference but I couldn't find a place in the documentation where it is described. and not escaping characters is one of the common mistake people do :) tested on the technical training yesterday ;)