[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
Maybe add a note/warning to the tutorial for newcomers, and one at the top of the generic data/XML files documentation (data.rst)?