New SASS guide

More than happy to hear about...

* Stuff I've missed
* Bad examples
* Poor English
* Problems with structure
* My awesomeness :?:
* ...
* Just plain questions

I expect to follow this up as the component library/styleguide becomes clearer. And as we roll out some KSS into the new Rails happy path.

Expect a presentation version of this too in short(ish) order.
Similar. But `em` not needed. In a world where `rem` is widely supported we'd be certainly having a discussion about that as a sizing tool. However it's easier, given we're sizing in `px` for `font-size`, just to not use `em` at all.