make github link easier to find

[katestange]

In the documentation, the github link should be less hidden. It took a while to find it while presenting.

[gwhitney]

Specifically, the "Overview" page (which I think/hope is just the README file?) should perhaps have one of those "Fork me on Github" banners, or if not, a link to the github source somewhere near the top.

While we are there, the "How do I use it?" section should be updated to indicate the latest release (currently alpha-level) is always available at numberscope.colorado.edu.