[DOC] Add Doc for new feature 2-phase search on NeuralSparseQuery

[conggguan]

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 and all versions that are affected.
I added a new feature in NeuralSearch Plugin which can be used to speed up the NeuralSparseQuery. As this is a new feature documentation needs to be added.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
RFC: opensearch-project/neural-search#646
PR: opensearch-project/neural-search#747