dc embedding updates

(EDIT: The problem behind this disclaimer has been fixed)

Big disclaimer up front: I clearly messed up with Git while working on this branch. It includes a ton of commits on files that I did not personally touch, or at least did not mean to make a part of this branch. So, I will need to find a way to fix that before this can be merged, and figure out how that happened so I can stop looking so foolish when making pull requests in the future. The main file I'd actually _like_ to make changes to with this pull request is `embedding.md`, plus a few other files related to raptorizing @thesarahmei's API embed video.

So with that in mind, @wistia/champs, please check this out and let me know whatcha think! Will this help people understand and use our embed codes more effectively? Is there anything you think needs more or less detail? Anything important missing?
feels like we should consolidate info into the Video SEO doc page!