docs: add Try It Sandbox badge links to top of README and index.html

Author: Corey B

README.md

@@ -2,6 +2,11 @@
 
 **FDD is to PDF what Git is to a printed code-review.** It’s alive, it’s versionable, and it’s secure.
 
+[![▶ Try the Live Sandbox](https://img.shields.io/badge/%E2%96%B6%20Try%20the%20Live%20Sandbox-38bdf8?style=for-the-badge)](https://spuds0588.github.io/open-fdd/site/sandbox.html)
+[![Read the Manifesto](https://img.shields.io/badge/Read%20the%20Manifesto-002147?style=for-the-badge)](https://spuds0588.github.io/open-fdd/site/index.html)
+[![npm](https://img.shields.io/badge/npm%20install-@coreyburns%2Fnode--fdd-cb0000?style=for-the-badge&logo=npm)](https://www.npmjs.com/package/@coreyburns/node-fdd)
+
+
 ## Vision
 OpenFDD aims to replace "visual-first" static documents (like PDFs) with "data-first" interactive containers that are human-readable, machine-verifiable, and self-persistent.