Deployments: Explain how GitHub App interacts with Deployments for GH tokens

[jkodroff]

We need to ensure the docs for both the GH App and Deployments itself explains the following:

From @flostadler: If the deployment is configured with a PAT (e.g. via the GitHub app) it should get promoted to a top level GITHUB_TOKEN env var automatically and "just work".

We need to audit any references to adding a GITHUB_TOKEN env var in Deployments and ensure that we include a note of explanation in those places that the GH App may make this unnecessary.