Fully document Mastodon's webhooks #1851

Mastodon has had webhook support since version 4.0

At the moment, there is only a short mention of these in the moderation section 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)