[OpenAPI] Add doc_tag to machine learning APIs

[lcawl]

The purpose of this PR is to subdivide the new machine learning API docs so they are more aligned with the groupings in the old API docs.

This is accomplished by using the new doc_tag to differentiate between the anomaly detection, data frame analytics, and trained model APIs.

@szabosteve if you prefer different tag names, display names, or external doc links, please feel free to override my choices here.

I also turned off the openapi-tags-alphabetical linting rule; it generated false errors due to our use of x-displayName.

[github-actions]

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

API	Status	Request	Response
ml.clear_trained_model_deployment_cache	🟢	1/1	1/1
ml.close_job	🟢	64/64	63/63
ml.delete_calendar_event	🟢	4/4	4/4
ml.delete_calendar_job	🟢	3/3	3/3
ml.delete_calendar	🟢	5/5	5/5
ml.delete_data_frame_analytics	🟢	2/2	2/2
ml.delete_datafeed	🟢	3/3	3/3
ml.delete_expired_data	🟢	5/5	5/5
ml.delete_filter	🟢	27/27	27/27
ml.delete_forecast	🟢	3/3	3/3
ml.delete_job	🟢	47/47	47/47
ml.delete_model_snapshot	🟢	2/2	2/2
ml.delete_trained_model_alias	🟢	3/3	3/3
ml.delete_trained_model	🟢	5/5	5/5
ml.estimate_model_memory	🟢	16/16	16/16
ml.evaluate_data_frame	🟢	15/15	15/15
ml.explain_data_frame_analytics	🟢	7/7	7/7
ml.flush_job	🟢	15/15	15/15
ml.forecast	🟢	1/1	1/1
ml.get_buckets	🟢	14/14	14/14
ml.get_calendar_events	🟢	15/15	15/15
ml.get_calendars	🟢	17/17	17/17
ml.get_categories	🟢	12/12	12/12
ml.get_data_frame_analytics_stats	🟢	12/12	12/12
ml.get_data_frame_analytics	🔴	17/17	14/17
ml.get_datafeed_stats	🟢	27/27	27/27
ml.get_datafeeds	🔴	20/20	9/20
ml.get_filters	🟢	13/13	13/13
ml.get_influencers	🟢	11/11	11/11
ml.get_job_stats	🔴	32/32	29/32
ml.get_jobs	🔴	31/31	22/31
ml.get_model_snapshot_upgrade_stats	🟢	3/3	3/3
ml.get_model_snapshots	🟢	18/18	18/18
ml.get_overall_buckets	🔴	16/16	7/15
ml.get_records	🟢	8/8	8/8
ml.get_trained_models_stats	🔴	17/17	10/17
ml.get_trained_models	🔴	36/37	32/37
ml.infer_trained_model	🟢	10/10	10/10
ml.open_job	🟢	83/83	83/83
ml.post_calendar_events	🟢	12/12	12/12
ml.post_data	🔴	9/11	13/18
ml.preview_data_frame_analytics	🟢	3/3	3/3
ml.preview_datafeed	🟢	17/17	17/17
ml.put_calendar_job	🟢	12/12	12/12
ml.put_calendar	🟢	135/135	135/135
ml.put_data_frame_analytics	🔴	32/33	32/33
ml.put_datafeed	🔴	70/71	53/71
ml.put_filter	🟢	27/27	27/27
ml.put_job	🔴	219/227	223/225
ml.put_trained_model_alias	🟢	13/13	13/13
ml.put_trained_model_definition_part	🟢	1/1	1/1
ml.put_trained_model_vocabulary	🟢	1/1	1/1
ml.put_trained_model	🔴	15/16	6/16
ml.reset_job	🟢	2/2	2/2
ml.revert_model_snapshot	🟢	2/2	2/2
ml.start_data_frame_analytics	🟢	1/1	1/1
ml.start_datafeed	🟢	24/24	24/24
ml.start_trained_model_deployment	🔴	14/14	4/14
ml.stop_data_frame_analytics	🟢	5/5	5/5
ml.stop_datafeed	🟢	17/17	17/17
ml.stop_trained_model_deployment	🟢	10/10	10/10
ml.update_data_frame_analytics	🟢	2/2	2/2
ml.update_datafeed	🔴	7/7	2/7
ml.update_filter	🟢	3/3	3/3
ml.update_job	🔴	4/5	5/5
ml.update_model_snapshot	🟢	3/3	3/3
ml.update_trained_model_deployment	🔴	4/4	2/4
ml.upgrade_job_snapshot	🟢	3/3	3/3

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

[szabosteve]

LGTM!