[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
Not sure. Ideally we'd split the tree and list completely (that's what the client does) instead of having this confused state where there's a tree view which usually isn't displayed as a tree, but could be.