[DOCS] Convert cat API examples to json format

[lcawl]

Relates to #4081

This PR updates the cat API examples to use the formt=json query parameter, since they were using the default text string format, which wasn't being correctly represented in the OpenAPI output.

[github-actions]

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

API	Status	Request	Response
cat.aliases	🟢	29/29	29/29
cat.allocation	🟢	12/12	12/12
cat.component_templates	🟢	3/3	3/3
cat.count	🟢	9/9	9/9
cat.fielddata	🟢	6/6	6/6
cat.health	🟢	3/3	3/3
cat.indices	🟢	33/33	32/32
cat.master	🟢	1/1	1/1
cat.ml_data_frame_analytics	🟢	5/5	5/5
cat.ml_datafeeds	🟢	4/4	4/4
cat.ml_jobs	🟢	4/4	4/4
cat.ml_trained_models	🟢	1/1	1/1
cat.nodeattrs	🟢	5/5	5/5
cat.nodes	🟢	11/11	11/11
cat.pending_tasks	🟢	1/1	1/1
cat.plugins	🟢	2/2	2/2
cat.recovery	🟢	6/6	6/6
cat.repositories	🟢	6/6	6/6
cat.segments	🟢	9/9	9/9
cat.shards	🟢	11/11	11/11
cat.snapshots	🟢	4/4	4/4
cat.tasks	🟢	11/11	11/11
cat.templates	🟢	17/17	17/17
cat.thread_pool	🟢	9/9	9/9

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

[github-actions]

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

API	Status	Request	Response
cat.aliases	🟢	29/29	29/29
cat.allocation	🟢	12/12	12/12
cat.component_templates	🟢	3/3	3/3
cat.count	🟢	9/9	9/9
cat.fielddata	🟢	6/6	6/6
cat.health	🟢	3/3	3/3
cat.indices	🟢	33/33	32/32
cat.master	🟢	1/1	1/1
cat.ml_data_frame_analytics	🟢	5/5	5/5
cat.ml_datafeeds	🟢	4/4	4/4
cat.ml_jobs	🟢	4/4	4/4
cat.ml_trained_models	🟢	1/1	1/1
cat.nodeattrs	🟢	5/5	5/5
cat.nodes	🟢	11/11	11/11
cat.pending_tasks	🟢	1/1	1/1
cat.plugins	🟢	2/2	2/2
cat.recovery	🟢	6/6	6/6
cat.repositories	🟢	6/6	6/6
cat.segments	🟢	9/9	9/9
cat.shards	🟢	11/11	11/11
cat.snapshots	🟢	4/4	4/4
cat.tasks	🟢	11/11	11/11
cat.templates	🟢	17/17	17/17
cat.thread_pool	🟢	9/9	9/9

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

[kilfoyle]

LGTM! 🍾