Well, `config.yaml` has been playbook-managed for a long time.
It's now extended to match the default sample config of the Discord
bridge.
With this patch, we also make `registration.yaml` playbook-managed,
which leads us to consistency with all other bridges.
Along with that, we introduce `./config` and `./data` separation,
like we do for the other bridges.
It didn't mention `matrix_appservice_discord_client_id` and
`matrix_appservice_discord_bot_token`, which makes it hard for
beginners.
Related to #105 (Github Pull Request).
