docs(readme): rewrite intro, add configuration guide, add proper licencing
- Rebrand project title and clarify that it’s a template, not a single site
- Add full “Configuration” section with env vars, slug mappings, and asset notes
- Improve getting started steps and fix numbered lists
- Clean up formatting (remove trailing spaces, consistent list markers)
- Expand platform documentation and usage examples
- Update licence, links, and troubleshooting sections for accuracy