Introduce 'Back to top' component and demo

[mcarrano]

Back to top is a floating button that, when clicked, scrolls the user back to the top of the page. The component should include a demo that shows how this behaves in the context of a page.

Core issue: patternfly/patternfly#4255

[mcarrano]

@jessiehuff can you take a look at this one from an a11y standpoint? The designs are attached to the core issue. We are wondering how the user would navigate to this via the keyboard.

[jessiehuff]

In terms of keyboard navigation, it will be in order of wherever it appears in the DOM. I imagine that in this case it would be important for us to place this at the bottom of whatever content that the user is scrolling back to the top of. I think we should include documentation about this as well.