Add Crowbar personas and use cases

Move content from http://crowbar.sync.in/crowbar-usecases to Git, so it's more accessible and visible to Crowbarians. Also makes it easier to be kept up to date.

[View rendered file](https://github.com/jamestyj/crowbar/blob/608c2ecea40bfaf1f7b5a39e20ef2c8b90728017/doc/devguide/use-cases.md).