Project: add GHA directions to project creation

[agjohnson]

#606 adds a button to help cover basic gaps in UX for GHA users, but we are seeing the need for a bit more guidance during this process. There are some cases described here we might want more opinionated UI for:

• GitHub App: link to app from import page #606 (comment)

[agjohnson]

There might be more room for improvements to the repository selection UI too. It's there that we might be able to show a list of org/users that the GHA is installed for, to prompt the user to add the GHA for additional organizations.