Link to kNN search guide in dense_vector docs (#91372)

Before it linked to script_score and approximate kNN separately, but now we have
a single page that describes both approaches. This change also removes a link to
the deprecated _knn_search API.

Author: jtibshirani

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

@@ -5,13 +5,8 @@
 <titleabbrev>Dense vector</titleabbrev>
 ++++
 
-The `dense_vector` field type stores dense vectors of float values. Dense
-vector fields can be used in the following ways:
-
-* In <<query-dsl-script-score-query,`script_score`>> queries, to score
-documents matching a filter
-* In the <<knn-search-api, kNN search API>>, to find the _k_ most similar
-vectors to a query vector
+The `dense_vector` field type stores dense vectors of numeric values. Dense
+vector fields are primarily used for <<knn-search,k-nearest neighbor (kNN) search>>.
 
 The `dense_vector` type does not support aggregations or sorting.