This commit adds detailed API docs for the ES|QL async apis, as well as moving the narrative to the ES|QL specific REST docs. Additionally, a few top-level lists are reflowed to reflect that there are now multiple APIs.
This commit adds two new endpoints to support ES|QL async querying.
The endpoints added are:
1. GET /_query/async - for initiating a query
2. GET /_query/async/{id} - for retrieving the result
