[OpenAPI] Edits the autoscaling APIs

[lcawl]

This PR edits the text to align with https://www.elastic.co/guide/en/elasticsearch/reference/master/autoscaling-apis.html.
It also changes the examples from .json to .yaml files and adds them to the overlays.

Review notes

Since the openAPI document is too big for previews and not hooked into buildkite previews yet, if reviewers are wanting to review the strings, they all appear in the output/openapi/elasticsearch-openapi.json file.

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
autoscaling.delete_autoscaling_policy	⚪	Missing test	Missing test
autoscaling.get_autoscaling_capacity	⚪	Missing test	Missing test
autoscaling.get_autoscaling_policy	⚪	Missing test	Missing test
autoscaling.put_autoscaling_policy	⚪	Missing test	Missing test

You can validate these APIs yourself by using the make validate target.