Documentation for ISY994 changes to sensors and events

[OverloadUT]

Description:
Documentation for the new features in the ISY994 platform:

1. Binary sensors are now automatically detected (including their device type) rather than needing to be named specifically based on the config
2. Leak and door sensors now have first-class support in Home Assistant, rather than having each raw ISY node show up as a separate Hass Entity.
3. Heartbeats from leak and door sensors now have first-class support in Hass - they show up as a battery sensor that turns On when a heartbeat is missed
4. Insteon "control" events now get emitted as Hass events, allowing automations based on button presses.

Pull request in home-assistant (if applicable): home-assistant/core#10805

Checklist:

• Branch: Fixes, changes and adjustments should be created against current. New documentation for platforms/components and features should go to next.
• The documentation follow the standards.

[amelchio]

I don't know Insteon but this looks good to me 👍