Add 2.15 to version history (#7484) (#7492)

_about/version-history.md

@@ -10,6 +10,7 @@ canonical_url: https://opensearch.org/docs/latest/version-history/
 
 OpenSearch version | Release highlights | Release date  
 :--- | :--- | :--- 
+[2.15.0](https://github.com/opensearch-project/opensearch-build/blob/main/release-notes/opensearch-release-notes-2.15.0.md) | Includes parallel ingestion processing, SIMD support for exact search, and the ability to disable doc values for the k-NN field. Adds wildcard and derived field types. Improves performance for single-cardinality aggregations, rolling upgrades to remote-backed clusters, and more metrics for top N queries. For a full list of release highlights, see the Release Notes. | 25 June 2024
 [2.14.0](https://github.com/opensearch-project/opensearch-build/blob/main/release-notes/opensearch-release-notes-2.14.0.md) | Includes performance improvements to hybrid search and date histogram queries with multi-range traversal, ML model integration within the Ingest API, semantic cache for LangChain applications, low-level vector query interface for neural sparse queries, and improved k-NN search filtering. Provides an experimental tiered cache feature. For a full list of release highlights, see the Release Notes. | 14 May 2024
 [2.13.0](https://github.com/opensearch-project/opensearch-build/blob/main/release-notes/opensearch-release-notes-2.13.0.md) | Makes agents and tools and the OpenSearch Assistant Toolkit generally available. Introduces vector quantization within OpenSearch. Adds LLM guardrails and hybrid search with aggregations. Adds the Bloom filter skipping index for Apache Spark data sources, I/O-based admission control, and the ability to add an alerting cluster that manages all alerting tasks. For a full list of release highlights, see the Release Notes. | 2 April 2024
 [2.12.0](https://github.com/opensearch-project/opensearch-build/blob/main/release-notes/opensearch-release-notes-2.12.0.md) | Makes concurrent segment search and conversational search generally available. Provides an experimental OpenSearch Assistant Toolkit, including agents and tools, workflow automation, and OpenSearch Assistant for OpenSearch Dashboards UI. Adds a new match-only text field, query insights to monitor top N queries, and k-NN search on nested fields. For a full list of release highlights, see the Release Notes. | 20 February 2024