docs: fix Quick Start to use local PLC/relay for development
The previous Quick Start used localhost:3000 as the PDS URL, which would
register a DID with the production PLC directory pointing to localhost.
Now uses docker compose to run local PLC and relay for safe local dev.
Also clarifies that setup should only be run for Node.js deployments
once the PDS is accessible at a public URL.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>