UI Behaviors guides

This first batch publishes 4 of the 45 upstream UI Behaviors guides; the rest are converted and will be released in later batches.

• Animate Elements To and From Top Layer — Animate elements such as dialogs, popovers, and tooltips as they’re entering/exiting the top layer.
• Build a Draggable Element — Support dragging to move a floating element around the page.
• Declarative Dialog and Popover Control — Toggle the visibility of a dialog or popover from a button without writing JavaScript.
• Scrollytelling — Animate visual properties on a target element — such as fading a backdrop, shifting a background color, or to create scrollytelling experiences — driven entirely by the scrollport position of a completely different element.