Add release notes 3.1 (#10140)

* Add release notes 3.1

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>

* Update

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>

---------

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>

Author: kolchfa-aws

release-notes/opensearch-documentation-release-notes-3.1.0.md

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+# OpenSearch Documentation Website 3.1.0 Release Notes
+
+The OpenSearch 3.1.0 documentation includes the following additions and updates.
+
+## New documentation for 3.1.0
+
+- MCP connector is GA in 3.1 [#10135](https://github.com/opensearch-project/documentation-website/pull/10135)
+- [Star-tree] Support for nested aggs & Removing experimental flag. [#10132](https://github.com/opensearch-project/documentation-website/pull/10132)
+- [MLCommons] Add details about memory in plan_execute_reflect agent [#10121](https://github.com/opensearch-project/documentation-website/pull/10121)
+- [MLCommons] Add details about metrics integration [#10120](https://github.com/opensearch-project/documentation-website/pull/10120)
+- Introduce memory optimized vector search (LuceneOnFaiss) in 3.1. [#10119](https://github.com/opensearch-project/documentation-website/pull/10119)
+- Document custom SSE endpoint parameter for MCP connectors [#10118](https://github.com/opensearch-project/documentation-website/pull/10118)
+- added ML agent update API [#10113](https://github.com/opensearch-project/documentation-website/pull/10113)
+- Update rescore support [#10108](https://github.com/opensearch-project/documentation-website/pull/10108)
+- Trace analytics update for 3.1 [#10107](https://github.com/opensearch-project/documentation-website/pull/10107)
+- Update documentation around  to show that it can be configured as a list to extract roles from nested JWT claims [#10103](https://github.com/opensearch-project/documentation-website/pull/10103)
+- [Pull-based Ingestion] Add pull-based ingestion limitations [#10096](https://github.com/opensearch-project/documentation-website/pull/10096)
+- Add new APIs documentation for MCP server feature [#10095](https://github.com/opensearch-project/documentation-website/pull/10095)
+- [Doc] Live Queries dashboard  [#10090](https://github.com/opensearch-project/documentation-website/pull/10090)
+- Add additional_config information [#10085](https://github.com/opensearch-project/documentation-website/pull/10085)
+- Update remote vector index build settings docs [#10084](https://github.com/opensearch-project/documentation-website/pull/10084)
+- Update neural stats API for 3.1 [#10079](https://github.com/opensearch-project/documentation-website/pull/10079)
+- Add documentation for semantic field. [#10078](https://github.com/opensearch-project/documentation-website/pull/10078)
+- Add collapse in hybrid query documentation [#10073](https://github.com/opensearch-project/documentation-website/pull/10073)
+- Add doc for neural sparse search with 2 new analyzers [#10062](https://github.com/opensearch-project/documentation-website/pull/10062)
+- Update docs for remote vector index build GA [#10061](https://github.com/opensearch-project/documentation-website/pull/10061)
+- Add SSE-KMS and bucket owner verification docs [#10060](https://github.com/opensearch-project/documentation-website/pull/10060)
+- Update AI Search Flows documentation [#10055](https://github.com/opensearch-project/documentation-website/pull/10055)
+- Introduce `fixed_char_length` algorithm in Text Chunking [#10043](https://github.com/opensearch-project/documentation-website/pull/10043)
+- Add Query Insights excluded_indices section [#10011](https://github.com/opensearch-project/documentation-website/pull/10011)
+- [Pull-based Ingestion] Update pull-based ingestion metrics and reset settings [#9993](https://github.com/opensearch-project/documentation-website/pull/9993)
+- Custom weights in rrf processor [#9922](https://github.com/opensearch-project/documentation-website/pull/9922)
+- Add back Rule Lifecycle API to main [#9875](https://github.com/opensearch-project/documentation-website/pull/9875)
+- Update Query Insights grouping setting names [#9799](https://github.com/opensearch-project/documentation-website/pull/9799)
+
+## In progress documentation for 3.1.0
+
+- Add forecasting section [#10133](https://github.com/opensearch-project/documentation-website/pull/10133)