Prompt user to use `gp rebuild` after updating their `.gitpod.yml` or Docker configuration.

[loujaybee]

There is no way for users to know to use gp rebuild as a way to iterate on their configuration. We should consider adding an in IDE / in editor prompt that asks a user if they'd like to test their new updated configuration. We should also consider adding this instruction to the default .gitpod.yml generated in inference, and also to the outputted .gitpod.yml from gp CLI.