[docs] Demo options as JSON

[oliviertassinari]

It's setting up the infrastructure needed for #9474. For instance:

A simple example of a scrollable image `GridList`.

{{"demo": "pages/demos/grid-list/ImageGridList.js", "hideEditButton": true}}

In practice, it's interpreted as JSON. Even if we decided to roll back the hideEditButton feature. It's still allowing extra configuration power for the future.

[lukePeavey]

I see what you meant now