Delete services-demo

[smatting]

This PR deletes the deploy/services-demo directory, including the demo.sh script.

I think it's not worth the upkeep costs (every service has 3 config files, e.g. services/brig/brig.integration.yaml, deploy/services-demo/confg/brig.demo.yaml,deploy/services-demo/confg/brig.docker.yaml) since it's largely unused by developers, who can use ./services/start-services-only.sh as an alterantive for testing purposes just as well.
For potential end users that want to run a demo of Wire, a helm-based deployment is much more suitable than demo.sh.
demo.sh --run-backoffice was needed to locally test stern. This PR adds stern to the services automatically spawned by start-services-only.sh.

Files and directories that are symlinking to deploy/services-demo are copied to their respective sources (find . -xtype l shows that no broken symlinks are created by this PR). .
Shell scripts in deploy/services-demo are moved to hack/bin.

All documentation is updated.

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines