Streamline README.md for open-source component focus

Summary

The README.md needs to be updated to:

• Be more concise and focused on the essentials
• Reflect that postgres_ai is an open-source building block within the larger PostgresAI ecosystem
• Direct users to https://postgres.ai for comprehensive documentation

Current State

The README is ~400 lines with detailed information that would be better served in dedicated documentation.

Proposed Changes

• Reduce README from ~400 lines to ~100 lines
• Add prominent "Quick links" section pointing to demo, docs, and console
• Keep essential info: features summary, architecture, quick start, requirements
• Move detailed troubleshooting, port lists, and testing docs to full documentation
• Add clear footer with PostgresAI branding and links