Fully document Mastodon's webhooks

Mastodon has had webhook support since version 4.0 At the moment, there is only a short mention of these in the [moderation section](https://docs.joinmastodon.org/admin/moderation/#report-events-webhook) in the docs. Some of these are documented, but coverage could be more complete. - explain that there's an admin page for creating a webhook / where to locate this - select the event (list supported events) and the address where it will be sent - define the payload (supports templating)