Separate design and user docs

Moved design documentation into a subdirectory.
Created some new user-focused documentation, and a glossary.
Partially resolves #1813
I didn't put the API change guidelines there.

As for the rest, I find it a PITA to submit a PR to add a single bullet item. I put the FAQs on the wiki so that I could immediately add FAQ bullets after answering questions on IRC. I put the Community and Media pages there for a similar reason -- when someone posted or Tweeted something, I could add a bullet. For the most part, this information isn't tightly coupled to the code. It's true that non-maintainers have no way of directly adding to the wiki, but they could file issues about the content.