fix(security): make the example env match the invite-only default

[maartenbode]

.env.example shipped SIGN_UP_ENABLED=true while config/fortify.php and the docs both document the default as false. Since composer setup copies the example file, manual installs silently opted into open registration. Docker installs were unaffected — the entrypoint never copies the example.

Documents pricore:install in the manual deployment and contributing setup steps, which previously relied on self-registration to create the first account.