Add information about what makes a webhook inactive / removed automatically

Based on a recent experience, I noticed that my webhook disappeared from the list of webhooks (`GET /webhooks`) every few days.

It turns out that if my service responds several times with a non-2XX HTTP status code, then after several such responses, the webhook will be removed.

This should be added to the documentation.

Start by locating the documentation for the GET /webhooks endpoint. Add the reported behavior that repeated non-2XX responses can make a webhook inactive and cause it to be removed; the work is done when this lifecycle information is clearly available to webhook users.

Written by the indexing model from the issue text.