Fixed #31802 -- Added system check for non-integer SITE_ID.

docs/ref/checks.txt

@@ -85,6 +85,7 @@ Django's system checks are organized using the following tags:
 * ``models``: Checks of model, field, and manager definitions.
 * ``security``: Checks security related configuration.
 * ``signals``: Checks on signal declarations and handler registrations.
+* ``sites``: Checks :mod:`django.contrib.sites` configuration.
 * ``staticfiles``: Checks :mod:`django.contrib.staticfiles` configuration.
 * ``templates``: Checks template related configuration.
 * ``translation``: Checks translation related configuration.
@@ -101,6 +102,10 @@ Some checks may be registered with multiple tags.
     The ``database`` checks are now run only for database aliases specified
     using the :option:`check --database` option.
 
+.. versionchanged:: 3.2
+
+    The ``sites`` tag was added.
+
 Core system checks
 ==================
 
@@ -813,6 +818,11 @@ The following checks are performed on any model using a
 * **sites.E002**: ``CurrentSiteManager`` cannot use ``<field>`` as it is not a
   foreign key or a many-to-many field.
 
+The following checks verify that :mod:`django.contrib.sites` is correctly
+configured:
+
+* **sites.E101**: The :setting:`SITE_ID` setting must be an integer.
+
 ``staticfiles``
 ---------------