Document is_receptive in the cluster agents API

What does this MR do and why?

The list, retrieve and create agent endpoints return an is_receptive field, and this page does not mention it anywhere. This adds it to the response attribute table of each of those three endpoints and to their four example responses.

I placed it after created_by_user_id, which is the order API::Entities::Clusters::Agent exposes the fields in, and kept the cell short enough that the existing table alignment does not have to change.

The agent token and receptive agent URL configuration examples on the same page are different entities, so they are untouched.

I found this while adding the same field to the Go API client, in gitlab-org/api/client-go!3043 (merged).

MR acceptance checklist

This change is documentation only. The example responses are valid JSON and no endpoint, parameter or behaviour changes.