Move content of design.gitlab to gitlab-ui

Pajamas and gitlab-ui have a lot of overlap, and refer mostly to the same things. Let's combine them!

Pajamas can live in this repo, either in the doc/ folder (we move/adapt the current content) or in a site/ folder.

With the docs generation stuff works we have a strange spread of content between gitlab-ui's components directories, and the content/ directory of design-gitlab-com repo. Keeping Pajamas content in this repo means:

1. one place for docs (no more half-filled-out readme.md in components folders!)
2. people actually test how their component looks on docs page
3. we are consistent with other component library structure (bootstrap, bootstrap-vue, storybook, bulma, & material-ui all do this)
4. easier issue tracking (a lot of design.gitlab issues depend on gitlab-ui changes)