[Banner] Render Banner as a region landmark labelled by a heading by default

[khiga8]

This is a recommendation that came out of accessible banner exploration.

Given that Banner is made visually prominent to sighted users through the use of icons and color, providing a heading and designating the Banner as a region landmark will significantly improve assistive technology users easily discover and navigate to the Banner. This recommendation is assuming that people aren't overusing Banners on a page.

However, making this a default would be a breaking change to the API for the PVC Banner. Therefore, I'm filing an issue for this to be considered in future investments.

For now, I've opted to update the documentation.