Restructure into standard mono-repo

It would be nice to restructure the design system into a standard mono-repo with a packages folder and use workspaces.

I don't think we can use the standard yarn/lerna combo because lerna doesn't let you publish a separate dist folder. Rush.js seems like it might be an appropriate alternative... although it's a little heavyweight.

Maybe we can just roll our own monorepo with just yarn?

I expect we'll have at least 2 packages:

• @ucam/design-system
• @ucam/design-system-docs

And possible future exensions:

• @ucam/design-system-gatsby-starter
• @ucam/design-system-assets - compressed and non-compressed images for download?