docs(renderer): improve README documentation for developers

Restructure and enhance the renderer package README with:
- Add table of contents for easy navigation
- Reorganize content into logical sections
- Add configuration tables with types and defaults
- Document all built-in plugins with usage examples
- Add guide for creating custom plugins
- Include package structure diagram
- Consolidate security documentation with API examples
- Update development instructions for pnpm monorepo
- Add supported embed platforms table
- Improve code examples with modern TypeScript syntax

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>