Documentation on building page banners has been added to the Extending

Review Board docs. This is in a new "building" section, which ultimately

other "how to build a thing" docs should move to.
These new docs cover all the information needed to build a page banner,

whether a simple one with a static message or a more complex one with

some interactivity.
A Storybook story has been added to allow interacting with the page

banner component in real-time.

Tested in Storybook.
Built the docs and checked for build errors, spelling errors, and bad

links.