[IMP] doc: warn gantt view is not available in community version

May be confusing for new users

Closes #17817

doc/howtos/backend.rst

@@ -1120,6 +1120,11 @@ default and behave as booleans (they can only be enabled by default).
 Gantt
 -----
 
+.. warning::
+
+    The gantt view requires the web_gantt module which is present in
+    :ref:`the enterprise edition <setup/install/editions>` version.
+
 Horizontal bar charts typically used to show project planning and advancement,
 their root element is ``<gantt>``.