[DOC] Documentation for new Neural Search OpenSearch plugin

[jmazanec15]

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Add documentation for new neural search plugin. More details on plugin can be found in RFC. Plugin uses ml-commons deep learning model inference support to produce dense vectors from text and then uses the k-NN plugin to make the vectors searchable and/or produce k-NN queries.

In the initial release, we are introducing a new ingestion processor (text_embedding) and query type (neural)

Release targeted as experimental for 2.4.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

1. Neural Search RFC(APIs/Usage/more details): [RFC] OpenSearch neural-search plugin neural-search#11
2. ml-commons related issue: [DOC]: Deep Learning Model for ML Commons #1486