Templates are private, readme uses the old build system for custom templates

The default templates (Nextjs Developer, etc.) are private and cannot be used by other users - although not sure if that's wanted behavior due to possible future updates of the templates itself which could then potentially affect all users of Fragments default templates.

Also the guide uses the old build system mechanism for building custom templates. I have created some changes https://github.com/bxxf/fragments/tree/docs/custom-templates-gen, but it would need some more changes to make it really dev-friendly (the README has been updated and there are scripts to help users automate the process of building their own templates based on the ones in the repo) - see it basically just as a sort of demo, I believe there must be a better way to approach this.

Start with the README and compare its custom-template guide with the changes in the linked docs/custom-templates-gen branch. Review how the default templates' private visibility should work, then establish a clear, supported workflow for building custom templates and update the documentation accordingly.