improve project documentation

[mr-tz]

• inspect / refine https://github.com/mandiant/capa/blob/master/doc/usage.md
• inspect/refine README and other documentation
• move some/all of our documentation to the website? Like, learning about the tool should be done on the website, and hacking on its code on Github? Definitely up for debate and interested in what people think (reorganize and extend readme #2258 (comment))

ref: #2258