Add Reference Index documentation

docs/advanced_topics/index.md

@@ -22,4 +22,5 @@ boundblocks_and_values
 multi_site_multi_instance_multi_tenancy
 formbuilder_routablepage_redirect
 streamfield_migrations
+reference_index
 ```

docs/advanced_topics/reference_index.md

@@ -0,0 +1,28 @@
+# Managing the Reference Index
+
+Wagtail maintains a reference index, which records references between objects whenever those objects are saved. The index allows Wagtail to efficiently report the usage of images, documents and snippets within pages, including within StreamField and rich text fields.
+
+## Configuration
+
+The reference index does not require any configuration. It will by default index every model, unless configured to prevent this. Some of the models within Wagtail (such as revisions) are not indexed, so that object counts remain accurate.
+
+The `wagtail_reference_index_ignore` attribute can be used to prevent indexing with a particular model or model field.
+
+-   set the `wagtail_reference_index_ignore` attribute to `True` within any model class where you want to prevent indexing of all fields in the model; or
+-   set the `wagtail_reference_index_ignore` attribute to `True` within any model field, to prevent that field or the related model field from being indexed:
+
+```python
+class CentralPage(Page):
+    ...
+    reference = models.ForeignKey(
+        "doc",
+        on_delete=models.SET_NULL,
+        related_name="page_ref",
+    )
+    reference.wagtail_reference_index_ignore = True
+    ...
+```
+
+## Maintenance
+
+The index can be rebuilt with the `rebuild_references_index` management command. This will repopulate the references table and ensure that reference counts are displayed accurately. This should be done if models are manipulated outside of Wagtail.