Add a workflow to leave comment in PR on successful update

[patcon]

Right now, the "test" comment shows what the change will be. But in order to confirm that the merged PR successfull updates the DNS, someone needs to know to hop into the "Actions" tab of the repo:

• merged PR (without confirmation): Added a go.talent.c4nada.ca shortlink service #34
• Action listing to see results of merge: https://github.com/g0v-network/domains/actions/workflows/domain-sync.yml

This would be better if there was a hint after merge. A simplest possible MVP is to just leave a comment with a link to the Actions listing page after merge: https://github.com/g0v-network/domains/actions/workflows/domain-sync.yml