Add reference bundle documentation (sulu#795)

* Add reference bundle docs

* Add review suggestion

* Add review suggestion

bundles/index.rst

@@ -23,6 +23,7 @@ documented the most important ones.
     page/index
     persistence
     preview/index
+    reference
     route/index
     search
     security/index

bundles/reference.rst

@@ -0,0 +1,68 @@
+ReferenceBundle
+===============
+
+The ReferenceBundle is tasked with tracking references among entities within the application.
+It enables developers and maintainers to quickly determine the relationships between entities and understand the manner
+and location in which an entity is utilized. Presently, the ReferenceBundle is capable of monitoring the usage of Snippets and
+Media within PHPCR entities such as `pages` and `snippets`. These references are managed distinctly for the draft
+state within the `admin context` and the live state within the `website context`.
+
+|
+
+The main reason we need this bundle is that, unlike traditional database references, our content management system
+operates on an unstructured data model. Therefore, we cannot rely solely on database references, which are usually preferred.
+It is essential to note that the ReferenceBundle should only be used for unstructured data, where database relations are
+not feasible.
+
+|
+
+Content maintainers are able to see the references to a specific entity in the `Insights` tab of an entity like `Snippet`.
+
+.. figure:: ../img/snippet-insights.png
+    :alt: Snippet References
+
+    Snippet References
+
+Refresh references
+------------------
+
+The references are automatically updated upon saving an entity. You also have the option to manually update the
+references by executing the `bin/console sulu:references:refresh` command. This command optionally accepts the
+<resource-key> argument. When this argument is provided, only the references for the specified resource key will be refreshed.
+
+.. code-block:: bash
+
+    bin/console sulu:references:refresh <resource-key>
+
+
+.. note::
+
+    Please note that references are only refreshed for the current context. To refresh the references for both the
+    admin and website contexts, you will need to execute the command twice via the `bin/adminconsole` and the `bin/websiteconsole`.
+
+Integrating references for custom content-types
+-----------------------------------------------
+
+To integrate the ReferenceBundle for custom content-types, you need to implement the `ReferenceContentTypeInterface` in your
+content-type class. The interface requires you to implement the `getReferences` method. The method already receives the
+`ReferenceCollector` which you can use to add references to the collector.
+
+|
+
+Example implementation for a custom content-type:
+
+.. code-block:: php
+
+        public function getReferences(PropertyInterface $property, ReferenceCollectorInterface $referenceCollector, string $propertyPrefix = ''): void
+        {
+            $data = $property->getValue();
+            if (!\is_array($data) || !isset($data['id'])) {
+                return;
+            }
+
+            $referenceCollector->addReference(
+                CustomEntity::RESOURCE_KEY,
+                (string) $data['id'],
+                $propertyPrefix . $property->getName()
+            );
+    }