Rethink the homepage's content

Right now it feels a little strange to be just dumped into the post categories with little explanation. What I'd like to do is:

• Explain what "a11y" means and point to this article. I think it's important to stress how "a11y" speaks specifically to web accessibility concerns.
• Point to the checklist as a good place to get started if you don't know where to begin (and possibly revise checklist content, as well).
• Surface other site content in the form of highlights. I'm thinking things like the latest post, featured members of the a11y community, new resources and events, that sort of thing.