[DOC] Add Documentation for Correlations in PPL

[brijos]

What do you want to do?

• Request a change to existing documentation (in PPL)
• 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 added correlation functionality to PPL in 2.13 and need it added to the documentation site for reference.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
https://github.com/opensearch-project/opensearch-spark/blob/main/docs/PPL-Correlation-command.md

[hdhalter]

Thanks, @brijos. Do you know who from Dev can create the PR?