Generate jx doc.go

[camh-]

cmd/jx/doc.go should be generated from the usage message output by jx --help.

1. Make jx --help output everything we want in the message
2. Add a //go:generate to generate doc.go from the usage message.

[camh-]

doc.go has a lot more in it than we would have jx --help output. Perhaps we should have doc_usage.go be the file that contains just the usage docs. It is also questionable whether to use //go:generate when we have a Makefile. The generate command will depend on the binary being built (although perhaps the generate command could be go run ./cmd/jx --help?).

We should add some help/usage text to the message at the very least.