Expanded documentation for Webhooks 2.0

Docs people: @fredlf @SvenDowideit 
Hub people: @kencochrane @fxdgear

Please review! Also note that this feature isn't in production yet but is slated for release in a few days.
I'm not sure what you mean by "context" here, especially since we refer to "build context" frequently throughout the docs. Can you clarify?