Improve design system concepts schema

[lukewarlow]

Following on from #845

Currently design system components have "concepts" and "anatomy". Concepts seems a rather broad term for what could be many different things. For example state pseudo classes (inc custom state ones), any corresponding UI behaviours associated with these pseudo class states, API methods, and attributes could all be "concepts". Is it worth us trying to segregate these a bit?

[lukewarlow]

This also begs the question stuff like pseudo class state can apply to pseudo elements how do we deal with this nesting?

[github-actions]

There hasn't been any discussion on this issue for a while, so we're marking it as stale. If you choose to kick off the discussion again, we'll remove the 'stale' label.