Issue Description:

The simplest case, and the one that will serve the vast majority of SPM users, is to create an application that uses other packages. But how to do that requires extracting it from a complicated 3-package example in the documentation, and `swift project init --type executable` is not easy to discover. `swift project --help` doesn't give an indication that I can type `swift project init --help` and get detailed help for the init command.

SR-12304 doesn't help, either ;-)

I recommend:

• The very first example should show how to create a simple executable package.

• the `--help` text should be amended with something like what's at the end of `git --help` output: “See 'git help <command>' or 'git help <concept>'
  to read about a specific subcommand or concept.”