Add Elasticsearch VDB provider to docs/providers/matris.md
The `ai_vdb_provider_elasticsearch` module (https://www.drupal.org/project/ai_vdb_provider_elasticsearch) implements the AI Search VDB interface against Elasticsearch's native `dense_vector` + HNSW. Currently, it is the only Drupal VDB provider offering native hybrid search (kNN + BM25 merged via Reciprocal Rank Fusion) in a single Elasticsearch request. Should be added to the matrix under the Vector Database column. [ai-matrix-add-elasticsearch.patch](/uploads/91488730bc49c8344ddecd6762bb14a2/ai-matrix-add-elasticsearch.patch)
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