Create contribute page

[wcaleb]

This will combine the current Submissions, Contact, and Get Involved page. We will replace "submissions" in the nab bar with "contribute."

[fredgibbs]

added new contribute page, linking to current submissions page. submissions page has too much detail for a generic contribute page; we might ultimately incorporate some of our styling conventions here as well. the contact and get-invovled pages are no longer used but i've left them in the root directory for now until the new pages are finalized.