[DOC] Add documentation for new feature ReIndex Workflow Step in Flow Framework

[owaiskazi19]

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.
We will be enabling easy migration of BM25 index to neual index through a new workflow step called as ReIndex workflow step in Flow Framework. Once opensearch-project/flow-framework#718 is merged, will create a doc PR for the same to add the documentation.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
opensearch-project/flow-framework#617

[minalsha]

@owaiskazi19 lets ensure that documentation clearly outlines guardrails that need to be highlighted for customer. Please work with @navneet1v on the guardrails. Thanks

[minalsha]

@hdhalter what are the next steps here? PR has already been raised by @owaiskazi19 last week. Thanks

[hdhalter]

Great! Looks like it has been approved from a technical standpoint, so I pushed it forward to doc review.