GA doc fixes for GTM and crappy images

Just added a section to address the trouble with Google Tag Manager, as well as updated the images and descriptions for how to actually look at that data in your GA account.

Two things I need a touch of feedback on:

- Does the GTM stuff make sense in context?
- Do you see any copy in there that needs better clarification?

Feels like assuming there's nothing horribly wrong, this can just be merged in.

Go to it @wistia/champs! :hand: 
Good call. Should definitely add a list.