feat: add next-best-practices documentation

- Create route-handlers.md to document API endpoint creation using route.ts files, including usage examples and supported methods.
- Add rsc-boundaries.md to outline detection rules for invalid patterns when crossing Server/Client component boundaries.
- Introduce runtime-selection.md to guide the use of Node.js vs Edge runtime in Next.js applications.
- Document script loading best practices in scripts.md, emphasizing the use of next/script for performance.
- Add self-hosting.md to provide instructions for deploying Next.js outside of Vercel, including Docker and PM2 deployment strategies.
- Create suspense-boundaries.md to explain the necessity of Suspense boundaries for certain client hooks.
- Add find-skills meta and SKILL.md files to introduce a new skill for discovering and installing agent skills.
- Create web-design-guidelines SKILL.md to provide a skill for reviewing UI code against Web Interface Guidelines compliance.

Author: ssdeanx