Update Diffusion main documentation

Summary:
This was really out of date and full of lies.

Mostly I've deleted sections, since the UI is way way more self-explanatory and much better at surfacing errors now.

Test Plan: L@@K

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D8873