Build guide: Connect from a front end

[pdaoust]

Show how to establish and reuse an app websocket connection from a JavaScript front end. Assume that:

1. The front end will be served up by a runtime that uses the APP_INTERFACE_PORT connection string injection technique and handles cap grant creation for you (i.e., the default situation when you use the scaffolding tool.
2. We don't know what front end framework the dev will be using (i.e., use vanilla JS)

[pdaoust]

Closed by #526