Stack demo, WIP

[mrienstra]

New demo to showcase how to change the stack order of objects on the canvas.

Doesn't cover grouped objects, multiple selections, etc., just objects directly added to the canvas, one at a time.

Thoughts?

Tempted to add more functionality, or have two demos in one, like the "Custom controls" demo -- the first could be even simpler than what I have now, and the second more complex.

Off topic of this PR: it would be really nice to have demo code available as both Typescript and Javascript!

[asturur]

i m still confused on how content should be organized.
You demo is an halfway throug some code and explaining the object stacking.
The available content on the website is still a copy of what was on fabricjs.com and i m not sure the direction is the good one.

[asturur]

Concepts needs to be grouped in small units that can be then summarized in a page with some code snippets.
I think one could be working with the stack and that and this demo could become that.
But i need to make at least some introduction pages to set the style / expectations.
I m also happy about content review once is there.