Document how to debug the GitHub app

[cnunciato]

Our GitHub app docs don't explain how the CLI and Service conspire to produce GitHub PR comments, which makes it hard to debug when comments aren't produced. We should explain that Pulumi examines the closest .git directory and adds commit metadata to the update, which the Service uses to find an associated PR to comment on.

[theodorvaleanu-vgw]

+1 to this, we are also having issues troubleshooting why the GitHub app is not picking up the previews.

[cnunciato]

Slack discussion that led to the filing of this: