Add a generic /api-demo to the codebase

[saltednut]

It can just show articles on the right and the JSON output on the left.

Similar to this kind of thing we added to the old demo.

[saltednut]

We may actually wanna ship this with the article content type. And not make it part of DFS Default per say, but most scenarios would have it turned on.

[saltednut]

This is pretty far along, was easy to copy from a previous demo and modify it. However, I am having issues getting the JS side of things working correctly and we need to theme the page to use two columns correctly.

[saltednut]

Current state

[saltednut]

[saltednut]

I think we can do better but this can work for the first take until @kevinfunk has time to come in and polish it up to use 2 columns and do more than just collapse. No rush, this current state should be good enough to do the demo, but having 2 cols would be better.

[kevinfunk]