Execution hint documentation added to cardinality agg

[maitreya2954]

Description

This PR adds documentation for execution_hint field that is being added for cardinality aggregator introduced in https://github.com/opensearch-project/OpenSearch/pull/15764. Waiting for main repo PR to be merged.

Issues Resolved

Closes #8264

Version

No idea

Frontend features

N/A

Checklist

• [x] By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and subject to the Developers Certificate of Origin. For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions[bot]]

Thank you for submitting your PR. The PR states are In progress (or Draft) -> Tech review -> Doc review -> Editorial review -> Merged.

Before you submit your PR for doc review, make sure the content is technically accurate. If you need help finding a tech reviewer, tag a maintainer.

When you're ready for doc review, tag the assignee of this PR. The doc reviewer may push edits to the PR directly or leave comments and editorial suggestions for you to address (let us know in a comment if you have a preference). The doc reviewer will arrange for an editorial review.

[asimmahmood1]

@maitreya2954 I'm wondering if you're still interested to get this merged in? If not, is ok if I take over this PR?

[kolchfa-aws]

@maitreya2954 Closing this PR in favor of https://github.com/opensearch-project/documentation-website/pull/9224