NGSTACK-805: add documentation

docs/reference/asynchronous_indexing.rst

@@ -0,0 +1,30 @@
+Asynchronous indexing
+=====================
+
+``Asynchronous indexing`` implements asynchronous indexing mechanism for Repository PHP API through Symfony's Messenger
+component. This solves a number of use cases where standard synchronous indexing mechanism fails, because it tries to
+execute indexing as part of the PHP API call. For example, hiding a large subtree though the Admin UI will fail with
+standard synchronous indexing implementation, while with asynchronous indexing enabled it will be processed in the
+background, without blocking the UI or causing timeouts.
+
+This also enables sane implementation of various custom use cases, for example indexing file's content, which might also
+require more time to execute and hence block the UI or cause timeouts.
+
+In order to enable asynchronous indexing, use the following configuration:
+
+.. code-block:: yaml
+
+    netgen_ibexa_search_extra:
+        use_asynchronous_indexing: true
+
+Together with the configuration above, you will need to configure the Messenger component so that messages are handled
+through the queue. Otherwise, the Messenger's mechanism will handle them them synchronously. An
+`example configuration <https://github.com/netgen/ibexa-search-extra/blob/master/bundle/Resources/config/messenger.yaml>`_
+for that is provided. If it fits your use case, use it, otherwise be free to implement your own.
+
+Additionally, you will need to start the consumer to process the queue. For the example configuration, you would do that
+with:
+
+.. code-block:: console
+
+    bin/console messenger:consume netgen_ibexa_search_extra_asynchronous_indexing --time-limit=1800 --limit=4096

docs/reference/map.rst.inc

@@ -1,4 +1,5 @@
 * :doc:`/reference/installation_instructions`
+* :doc:`/reference/asynchronous_indexing`
 * :doc:`/reference/location_query`
 * :doc:`/reference/subdocuments`
 * :doc:`/reference/spellcheck_suggestions`