First time onboarding via the CLI

[schacon]

The first time you run the but command on a system, help the user by showing some onboarding info.

For a project that is getting initialized, tell them that you've chosen a target branch, or prompt them for which one they want. Tell them how to change it later.

Add a welcome message to the top of the first output that links to our documentation and suggests some next commands.

Inform them that we're changing their branch (if we do) and how to get out of "gitbutler mode"