Improve layout for examples on the demo page

[jamesnw]

The elements grid area for examples is a bit unwieldy to work within, and has some issues on mobile devices. Based on the examples we now have, I think these changes would be useful-

• The left and right margins aren't needed, especially on narrower screens.
• The elements grid area should scroll left and right, to prevent overflow
• This scrolling may need to happen independently of the scroll area that is used for position fallback examples
• The anchor element should be min-content- it currently is 100% width.
• By default, the anchor should be centered within elements
• elements should have sufficient padding to allow for a single absolutely positioned target touching the anchor (or alternatively, is there a way for elements to auto-grow so that the content box includes the absolutely positioned targets?)
• For cases with multiple anchors, it would be nice to have an easy way to specify the elements box height