Update _im-plugin/ism/api.md to include GET Policies Request for ISM

[m-a-x-e-d]

Description

We noticed that there is an undocumented request in the ISM plugin for listing all existing ISM policies. So far the only documented GET request for policy is:

GET _plugins/_ism/policies/policy_1

We would like to add the GET policies request to the documentation. Here are the relevant REST methods:

• https://github.com/opensearch-project/index-management/blob/main/src/main/kotlin/org/opensearch/indexmanagement/indexstatemanagement/resthandler/RestGetPolicyAction.kt
• https://github.com/opensearch-project/index-management/blob/main/src/main/kotlin/org/opensearch/indexmanagement/indexstatemanagement/transport/action/getpolicy/GetPoliciesRequest.kt
• https://github.com/opensearch-project/index-management/blob/main/src/main/kotlin/org/opensearch/indexmanagement/common/model/rest/SearchParams.kt

Issues Resolved

No issue was opened

Version

all

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]

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.

[kolchfa-aws]

@m-a-x-e-d Thank you for submitting this update! @vikasvb90 Could you review this PR?