[DOC]Create documentation for GET /_ingest/processor/grok API

[kolchfa-aws]

What do you want to do?

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

Tell us about your request. Provide a summary of the request and all versions that are affected.
GET /_ingest/processor/grok

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

[vagimeli]

@kolchfa-aws Is this related to #4193? If so, I'll add it to the work I'm doing now.

[kolchfa-aws]

Yes, this is an API to retrieve the patterns of the grok ingest processor.