Rejigger the whole WordPress doc

This all started because when WordPress launched version 4.0 the other day, [this customer](http://www.designbigger.com/blog/seo/adding-wistia-oembed-provider-wordpress-4-0/) reached out to tell us there was a fancy new way to enable oEmbed support for Wistia videos. About 90% of the way into all the changes I ended up making here, I realized that this customer was wrong. That's exactly how the Wistia WordPress plugin already does it, and nothing has really changed. But now we have a refreshed, cleaned up WordPress doc page anyway. Woo!

@wistia/champs, please take a second to check this version out when you can. I did a pull request here instead of pushing straight to master since it's quite a few changes.
This note is really funny and has good happy energy, but I'm not sure it's clear enough what they need to do. I think we should hit the "you need to use a Wistia subdomain for your address" a bit clearer.