[DOC] Document new normalization (z_score) and combination (rrf) techniques in HYBRID_OPTIMIZER experiments

[RexyZhang2001]

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The Search Relevance Workbench Hybrid Optimizer (HYBRID_OPTIMIZER) experiment now supports two additional techniques: z_score normalization and rrf (Reciprocal Rank Fusion) combination.

The following documentation page calls out an explicit list of supported techniques and currently omits the new ones:
https://docs.opensearch.org/latest/search-plugins/search-relevance/optimize-hybrid-search/#experimentation-process

Version:
v3.7.0

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
Related PRs:
opensearch-project/search-relevance#465
opensearch-project/dashboards-search-relevance#836