Zero-config `yarn etwin start`

When installing the @eternal-twin/website package inside another project, you need to manually create an etwin.toml configuration file before starting the website. This adds friction to the installation process.

We should provide a zero-config installation: when running without a config file, the CLI should use a default etwin.toml and warn the user that the config is missing. Most users would still need a configuration file, but it should at least improve the first start experience.