Add warnings in the doc about the deprecation of treeview.

Well, kind of deprecation. Tell the users that we expect them to use smartview instead.

Author: jordibc

doc/reference/reference_treeview.rst

@@ -1,6 +1,19 @@
 .. module:: treeview
 .. currentmodule:: ete4.treeview
 
+.. note::
+
+  Since ETE 4, the treeview module is **not** the preferred way to
+  create visualizations. This module is a port of the old
+  visualization in ETE, and we have maintained it, but recommend
+  instead to use the web visualization (:class:`smartview module
+  <ete4.smartview>`).
+
+  This module has less functionality and will not be generally
+  updated. Also, please note that the way it uses faces and layouts is
+  incompatible with the way smartview uses them.
+
+
 Treeview (qt graphics)
 ======================
 

doc/tutorial/tutorial_treeview.rst

@@ -1,5 +1,18 @@
 .. currentmodule:: ete4.treeview
 
+.. note::
+
+  Since ETE 4, the treeview module is **not** the preferred way to
+  create visualizations. This module is a port of the old
+  visualization in ETE, and we have maintained it, but recommend
+  instead to use the web visualization (:class:`smartview module
+  <ete4.smartview>`).
+
+  This module has less functionality and will not be generally
+  updated. Also, please note that the way it uses faces and layouts is
+  incompatible with the way smartview uses them.
+
+
 Tree drawing (Qt)
 =================