[DOCS] Add .NET client link to Bulk API docs (#57095)

This commit adds a link in the Bulk API documentation to
the .NET client's BulkAllObservable type and associated methods.

docs/reference/docs/bulk.asciidoc

@@ -109,6 +109,9 @@ JavaScript::
 
     See {jsclient-current}/client-helpers.html[client.helpers.*]
 
+.NET::
+    See https://www.elastic.co/guide/en/elasticsearch/client/net-api/current/indexing-documents.html#_multiple_documents_with_bulkallobservable_helper[`BulkAllObservable`]
+
 [float]
 [[bulk-curl]]
 ===== Submitting bulk requests with cURL