docs: Complete README.md rewrite for clarity and engagement

Rewrote the main README.md to be simpler, more exciting, and easier for new developers to understand.

Key changes include:
- More engaging introduction and welcoming tone.
- New "Quick Start" section for a fast first experience.
- Reorganized and clarified installation instructions for npm, CDN, and Deno.
- Updated "Explore Our JavaScript Toolkit for AI" section with current package names (e.g., @huggingface/agents), clearer descriptions, and links to individual package READMEs and npm pages.
- Refined code examples for better readability and added more explanatory comments.
- Ensured overall structure is logical and content is up-to-date.

Note: Project-level format, lint, and test scripts are currently failing due to unrelated setup/dependency issues (missing prettier-plugin-svelte, ESLint v9 config, vitest not found). These do not affect the README changes.

Author: google-labs-jules[bot]