Update sample-project.md

[codyaray]

Just some feedback from a quick review.

1. Make the ccloud CLI instructions match the CCloud UI instructions (i.e., only for service account id instead of all 3)
2. Simplify the CLI output with a grep command
3. Add more detail to the UI for creating cluster API Keys
4. Make variables required (so that they're prompted for if not provided)
5. Don't mark var.kafka_api_key as sensitive (since the key id can be logged, but the secret can't)
6. Fix guide to document how to pass variable values (as terraform.tfvars for now, but with disclaimer)