Add machine learning API examples

[lcawl]

Relates to #2144

The goal is that these examples would be added to the openAPI specification per https://spec.openapis.org/oas/latest#request-body-examples and https://spec.openapis.org/oas/latest#responses-object-example.

The examples are obtained from https://www.elastic.co/guide/en/elasticsearch/reference/current/ml-close-job.html, https://www.elastic.co/guide/en/elasticsearch/reference/current/ml-put-job.html, https://www.elastic.co/guide/en/elasticsearch/reference/current/ml-delete-job.html, and https://www.elastic.co/guide/en/elasticsearch/reference/current/ml-open-job.html, which are pages that will no longer exist when we start generating docs from the API specifications.

For now I've only added examples for "200" responses (since that's what we've got in the docs), but in case we want to provide examples for other response codes in the future, I've put that code in the folder name. If there's a better location or method for adding these examples, let me know!

[github-actions]

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

API	Status	Request	Response
ml.close_job	🟢	64/64	63/63
ml.delete_job	🟢	47/47	47/47
ml.open_job	🟢	83/83	83/83
ml.put_job	🔴	219/227	223/225

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