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: 
Could this description of GTM be a little more clear? Suggested edit: 

"Google Tag Manager is a tool created by the folks over at Google that makes it easy to manage the HTML tags for tracking and analytics on your website – no more tweaking a bunch of code on every page of your website manually when you need to make a change. Hooray!

While Google Tag Manager is _great_ for that reason, it does require some additional setup that's different from the workflows described above."