[DOC] Document the force_merge API

[andrross]

What do you want to do?

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

Tell us about your request.

The documentation is relevant for all versions of OpenSearch going back to 1.0.

We have some documentation of force merge for the index management plugin:

• https://opensearch.org/docs/latest/dashboards/im-dashboards/forcemerge/
• https://opensearch.org/docs/latest/im-plugin/ism/error-prevention/index#force_merge

However, we should have comprehensive documentation for the underlying REST API, e.g. POST /<index>/_forcemerge. A subject matter expert will need to write up this documentation, but I'm creating this placeholder issue for now.

What other resources are available?

Issue 7644 in OpenSearch changes some subtle behavior of one option of this API. I couldn't find existing documentation for this API to update, so we should probably create it :)