[IMP] base,doc: add scales attribute to gantt view

closes odoo/odoo#31210

doc/reference/views.rst

@@ -1355,6 +1355,9 @@ take the following attributes:
   * ``week``
   * ``month``
   * ``year``
+``scales``
+  comma-separated list of allowed scales for this view. By default, all scales
+  are allowed. For possible scale values to use in this list, see ``default_scale``.
 
 ``templates``
   defines the :ref:`reference/qweb` template ``gantt-popover`` which is used

odoo/addons/base/rng/gantt_view.rng

@@ -23,6 +23,7 @@
             <rng:optional><rng:attribute name="total_row"/></rng:optional>
             <rng:optional><rng:attribute name="collapse_first_level"/></rng:optional>
             <rng:optional><rng:attribute name="offset"/></rng:optional>
+            <rng:optional><rng:attribute name="scales"/></rng:optional>
 
             <rng:optional>
                 <rng:attribute name="default_scale">