build out splash page

[mcfrank]

Medium priority item to be discuss at some point with @vmarchman at some point.

minimally it might be good to reformat, e.g. top bar plus white background (or something a little easier to read black text on), screenshot?

and put a little bit more detail about:

• the system (e.g., github link)
• what languages and forms we have (could render dynamically, that'd be good)
• usage stats (only if it's easy, but would be fun to have)
• the EU mirror (contact info for EU folks to get started)
• the preprint