Document /ping endpoint

## Prerequisites

* [x] I am running the latest version. (`up upgrade`)
* [x] I searched to see if the issue already exists.
* [x] I inspected the verbose debug output with the `-v, --verbose` flag.
* [x] Are you an Up Pro subscriber?

## Description

Going to `/ping` sends a response of `healthy`. When an app already has a `/ping` endpoint, the request does not reach it and is intercepted by Up (I assume, not CF or APIG?)

Would be good to document this as I can't find it anywhere, and add the ability to disable/change it in the config?