Video SEO doc update

woooHooooo! Our Video SEO doc is one of the premier destinations for Video SEO information on the net, competing *mostly* with our own Learning Center page for the :crown:.

It could totally use some love! Thanks to Mr. Phil Nottingham, I started putting some notes together on factual inconsistencies or improvements we could make to the doc as it was.

One major change - I added the Learning Center video to the top of the page. This was a major Phil suggestion. I'll need some input from @ezrafishman and @notalyce before we push this one.

In the course of making this update, there were *also* a bunch of little tweaks that had been annoying me for a while. We have those cool `tip` and `note` blocks, but we couldn't use markdown in them. Now you can! :fire: 

With @liatwerber recent style changes, images and videos can now be larger. I removed the `image_resize` parameter we were running on the images, so now they look even better, and great on any screen :thumbsup: 

Still lots to go, but feel free to poke around and make suggestions.

@wistia/champs