Add push notification docs

[mmaatttt]

Docs for:

1. Automatic push config
2. Manual push config (also a tutorial formatted doc)
3. Rich push config
4. Push debugging

I put manual config in a separate doc to make the happy path of using automatic config a bit more prominent and to make the doc look a lot less daunting at first glance.

The automatic config doc is listed at the top level with "Core Functionality" and the other detail docs are nested with the "Push Notifications" topic of Appcues docs:

We can also decide to move some of this info to docs.appcues.com if we want, but we've gotta start writing it down somewhere.