[DOC] Alias exists API and Get Alias API are missing

[gaobinlong]

What do you want to do?

• [ ] Request a change to existing documentation
• [x] Add new documentation
• [ ] Report a technical problem with the documentation
• [ ] Other

Tell us about your request. Provide a summary of the request.

We have Alias Exists API and Get Alias API, but the documentation are missing.

*Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.

All versions.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

[gaobinlong]

@hdhalter Please assign this issue to me, thanks!

[hdhalter]

Thanks so much! We are making changes to the API template, so please take a look at the latest: https://github.com/opensearch-project/documentation-website/blob/main/templates/API_TEMPLATE.md. Tagging @Naarcha-AWS, as he is working on API content and will likely do the doc review.