Add search_only ISM action documentation

[lawofcycles]

Description

Add documentation for the new search_only ISM action introduced in index-management#1560.

The search_only action sets an index to search-only mode by calling the Scale API.

Changes:

• Added search_only to the list of ISM supported operations
• Added documentation section for the search_only action

Issues Resolved

Related to index-management#1560

Version

3.5 and later

Frontend features

N/A

Checklist

• 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]

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[kolchfa-aws]

Hey @lawofcycles Let me know when this PR is ready for a doc team review. Thanks!

[lawofcycles]

@kolchfa-aws Thank you for your attention. Now ready for review.

[kolchfa-aws]

Thank you, @lawofcycles! Doc review complete. Moving on to editorial review.

[natebower]

LGTM