[DOC] Adding RRF to hybrid search documentation

[ryanbogan]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request.
Existing documentation page for hybrid query in neural-search (https://opensearch.org/docs/latest/search-plugins/hybrid-search/) should be updated, the new addition should mention that hybrid query will support RRF (reciprocal rank fusion). RRF is a new processor for hybrid search that normalizes results based on ranks instead of the raw scores.
Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
Releasing in 2.19

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
Feature request issue: opensearch-project/neural-search#659
RFC: opensearch-project/neural-search#865
PR with main implementation: opensearch-project/neural-search#874