Simplify README features and setup

[Mine77]

What changed

• remove the Metrics section
• reduce Features to Editable, Perfect export, and Prompt Library
• link Prompt Library to the live StarryKit Explore page
• remove the Manual setup table from the project homepage
• point the header and install copy to a concise bilingual docs index

Validation

• npm test (5/5 passing)
• https://starrykit.com/explore returns HTTP 200
• git diff --check