Minor ann docs update (elastic#94783)

Replace the link to the deprecated knn search API and
added a link to the nightly benchmarks in Rally.

docs/reference/how-to/knn-search.asciidoc

@@ -44,6 +44,9 @@ For example your index might also include text fields and numerics, which also
 benefit from using filesystem cache. It's recommended to run benchmarks with
 your specific dataset to ensure there's a sufficient amount of memory to give
 good search performance.
+You can find https://elasticsearch-benchmarks.elastic.co/#tracks/so_vector[here]
+and https://elasticsearch-benchmarks.elastic.co/#tracks/dense_vector[here] some examples
+of datasets and configurations that we use for our nightly benchmarks.
 
 [discrete]
 include::search-speed.asciidoc[tag=warm-fs-cache]

docs/reference/mapping/types/dense-vector.asciidoc

@@ -60,9 +60,14 @@ similarity.
 
 In many cases, a brute-force kNN search is not efficient enough. For this
 reason, the `dense_vector` type supports indexing vectors into a specialized
-data structure to support fast kNN retrieval through the
-<<knn-search-api, kNN search API>>. You can enable indexing by setting the
-`index` parameter:
+data structure to support fast kNN retrieval through the <<search-api-knn, `knn` option>> in the search API
+
+NOTE: Indexing vectors for approximate kNN search is an expensive process. It
+can take substantial time to ingest documents that contain vector fields with
+`index` enabled. See <<tune-knn-search,k-nearest neighbor (kNN) search>> to
+learn more about the memory requirements.
+
+You can enable indexing by setting the `index` parameter:
 
 [source,console]
 --------------------------------------------------
@@ -86,10 +91,6 @@ PUT my-index-2
 efficient kNN search. Like most kNN algorithms, HNSW is an approximate method
 that sacrifices result accuracy for improved speed.
 
-NOTE: Indexing vectors for approximate kNN search is an expensive process. It
-can take substantial time to ingest documents that contain vector fields with
-`index` enabled.
-
 Dense vector fields cannot be indexed if they are within
 <<nested, `nested`>> mappings.