NGSTACK-836 add documentation for descendant indexing

Author: kmiocic

docs/reference/descendant_indexing.rst

@@ -0,0 +1,71 @@
+Descendant indexing
+=====================
+
+This feature helps in indexing hierarchical content structures. It allows the children of a content item to be indexed
+within the same document as the parent if both are configured for descendant indexing.
+
+''Configuration''
+
+To enable this feature, set up the descendant indexing configuration:
+
+.. code-block:: yaml
+    hierarchical_indexing:
+        descendant_indexing:
+            enabled: false
+            map:
+                content_type_identifier:
+                    handlers:
+                        - handler_identifier_1
+                        - handler_identifier_2
+                    children:
+                        content_type_identifier:
+                            indexed: true
+
+The ``enabled`` field must be set to true to activate descendant indexing services by registering them in the container.
+In the array parameter ``map`` we define the structure of content to be included in descendant indexing by content types.
+Any structure in the content tree that matches the configuration will be part of descendant indexing. Content can be
+part of the structure but not included in the index. To index the content in the parent document, set the ``indexed```
+parameter to ``true``.
+
+This feature is automatically triggered during indexing when configured correctly.
+
+Depending on what we want to index, we use different handlers. They represent the field mappers used to index the content.
+If you want to index content to the full text fields, you should use the 'ng_descendant_indexing_fulltext' handler:
+
+.. code-block:: yaml
+    hierarchical_indexing:
+        descendant_indexing:
+            enabled: true
+            map:
+                content_type_identifier:
+                    handlers:
+                        - ng_descendant_indexing_fulltext
+                    children:
+                        content_type_identifier:
+                            indexed: true
+
+To index something other than full text fields (e.g., location information or content metadata), implement new field
+mappers by extending the corresponding ``BaseFieldMapper`` and registering the field mapper as a service with needed tag.
+The ``getIdentifier()`` method returns a string of handler identifier which should match the handler
+identifier defined in the configuration.
+
+.. code-block:: php
+    public function getIdentifier(): string
+    {
+        return 'ng_descendant_indexing_fulltext';
+    }
+
+''AncestorIndexer''
+
+AncestorIndexer is a service that ensures descendant indexing is considered during reindexing. For example, if we edit
+content that is part of the descendant indexing map, the descendant content in which it is indexed should also be
+reindexed.
+
+The service contains methods ``indexSingle()`` and ``indexMultiple()``, which are called in handlers for any content
+changes (e.g., ``CopyContentHandler``, ``DeleteContentHandler``). These methods use the AncestorResolver service to
+resolve the ancestor to be reindexed. If no ancestor matches the configuration map structure, the ``resolveAncestor()``
+method returns null.
+
+The AncestorResolver service uses AncestorPathGenerator service to read from the configuration and return an array of
+strings representing all of the paths matching the given configuration in order to be able to find a match with any of
+the paths.