Make a user guide + project splash page

[anishathalye]

It could be nice to have a splash page (something that's more substantial than the readme). We could also have a detailed user guide there with detailed setup instructions, etc.

Having improved documentation could help with users not knowing how to use software. For example, I've gotten feedback where people have said that they don't know how to deal with projects that have moved locations - we have thought about this, and we support changing locations, but I guess people don't always get around to reading https://github.com/anishathalye/gavel#admin-panel-features