CRM-19813 - Document support for hooks via Symfony

Created by: totten

This PR adds a new section under "Hooks => Setup => Hooks with Symfony" to explain the integration between EventDispatcher and hooks in v4.7.19+.

Note that these docs are framed from a Hooks-First perspective -- paraphrasing, "Hooks are the main documented event-system, and Symfony EventDispatcher is just one face of that, and EventDispatcher only works in v4.7.19+." This frame is ahistorical (in that the EventDispacher was originally introduced by v4.5.0, and it wasn't used with hooks.) However, I think it's easier for our dev community to relate to the Hooks-First perspective.

For more sophisticated readers who care about EventDispatcher in earlier versions (v4.5.0-v4.7.18), I've included a brief "History".

Created by: seancolsen

This is really nice. So happy to see new docs like this come along with changes to core! Thank you @totten