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

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. 