feat: 5-minute quickstart — from zero to browsable catalog

[rajnavakoti]

Description

Create a streamlined onboarding experience: a user should be able to go from clone to a working catalog with their own data in under 5 minutes. This includes an init script or interactive wizard that scaffolds a minimal registry from scratch.

Acceptance Criteria

• Single command to scaffold a fresh registry (e.g. `npm run init` or `python scripts/init_registry.py`)
• Wizard asks: project name, 1-2 domain names, and generates minimal registry entries
• Generated catalog builds and renders correctly
• README quickstart section updated with the 5-minute path
• Works on macOS and Linux

Out of Scope

• Windows support (best-effort, not a blocker)
• Full registry population — just enough to see the catalog working

[rajnavakoti]

Done: splash page has 3-step quickstart, installation page updated with full walkthrough