gh-145299: Move __cached__ deprecation to its own section

The deprecation notice for __cached__ was incorrectly placed under
the __file__ attribute documentation. This moves it to a new
__cached__ section where it belongs.

Author: gourijain029-del

Doc/reference/datamodel.rst

@@ -1084,6 +1084,10 @@ this approach.
    :ref:`import system <importsystem>` may opt to leave it unset if it
    has no semantic meaning (for example, a module loaded from a database).
 
+.. attribute:: module.__cached__
+
+   The pathname of the compiled version of the module's code, if available.
+
    .. deprecated-removed:: 3.13 3.15
       Setting ``__cached__`` on a module while failing to set
       :attr:`!__spec__.cached` is deprecated. In Python 3.15,