Create new user intro page to link from banner

[mcking65]

Related to #28, develop content for an intro page for new users that can be linked from the banner.

[shawna-slh]

It would be good to have this for launch. If our promotion is successful, you will get a lot of new users visiting this resource. It would be a shame not to have a clear "about" for new visitors. I apologize that I don't remember the response in our meeting today to the straw proposal to have the About page be a few short sentences with a link to WAI-ARIA Overview

@mcking65 @a11ydoer