[1.10.x] Fixed #27355 -- Added “Creating extension using migrations” docs.

Backport of 4de8aaf7ff from master

docs/ref/contrib/postgres/fields.txt

@@ -261,27 +261,8 @@ A more useful index is a ``GIN`` index, which you should create using a
     To use this field, you'll need to:
 
     1. Add ``'django.contrib.postgres'`` in your :setting:`INSTALLED_APPS`.
-    2. Setup the hstore extension in PostgreSQL before the first ``CreateModel``
-       or ``AddField`` operation by adding a migration with the
-       :class:`~django.contrib.postgres.operations.HStoreExtension` operation.
-       For example::
-
-            from django.contrib.postgres.operations import HStoreExtension
-
-            class Migration(migrations.Migration):
-                ...
-
-                operations = [
-                    HStoreExtension(),
-                    ...
-                ]
-
-       Creating the extension requires a database user with superuser
-       privileges. If the Django database user doesn't have superuser
-       privileges, you'll have to create the extension outside of Django
-       migrations with a user that has the appropriate privileges. In that
-       case, connect to your Django database and run the query
-       ``CREATE EXTENSION IF NOT EXISTS hstore;``
+    2. :ref:`Setup the hstore extension <create-postgresql-extensions>` in
+       PostgreSQL.
 
     You'll see an error like ``can't adapt type 'dict'`` if you skip the first
     step, or ``type "hstore" does not exist`` if you skip the second.