New emulation docs

These are the docs I wrote up over the weekend on the new emulation mode additions in Canary. (Just *slightly* outdated in some parts now, will be updating this evening with a fresh Chromium build.)

So, one question probably being asked is, Why create a new page compared to updating the mobile-emulation page? I think that developers should get in the habit of simply emulating different devices. We need to test touch inputs on desktop sites, as well as different network speeds. Why title something so that it gets thought of as something that only applies to mobile? Hopefully tweaking the title a bit can encourage better practices.

If this is pulled in, then some other things need to be done.
* Update the navbar of DCC to link to Device Mode in place of the current Mobile Emulation.
* A redirect would need to be put in place from mobile-emulation to device-mode.
* The old content should be removed from the repo.

Once the navbar update is done, I can do the other two in a single PR.


So, assuming this is all good are there any thoughts on the content itself? Or any major things I am missing?