Documentation for how to develop a new Connection

This should go into the details of:
- How to build a basic connection (and what a connection is at a high level)
- How to subscribe to events from webhook sources
- How to add new webhook sources

This doesn't need to go into how to add non-webhook things which is a bit of advanced class right now, but should leave the door open for more docs.

No files or tests are named. Start by locating the existing connection and webhook-source entry points, then read how a basic connection is defined and how webhook events are subscribed to; done means documenting those flows and the process for adding a webhook source while explicitly leaving non-webhook extensions out of scope.

Written by the indexing model from the issue text.