Create user guide documentation for squash

Learned from Jonathan's talk that even a simple prototype requires a user documentation, indeed we had several questions from DM users that should be answered by the system itself

http://sqr-009.lsst.io/en/latest/ is linked from SQuaSH initial page but it is a design documentation, now that the code is in developement we should replace that documentation with a user guide that can be linked from the application.