feat(docs): add JavaScript/TypeScript SDK quickstart

Replace the placeholder JS/TS quickstart with a full guide mirroring
the Go quickstart structure: prerequisites, project setup, basic
encrypt/decrypt, ABAC policy management, and a complete reference
implementation.

Update the SDK quickstart index card from "coming soon" to active link.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: marythought

docs/sdks/quickstart/index.mdx

@@ -76,7 +76,7 @@ OpenTDF provides native SDKs in three languages. Choose your language to get sta
       </div>
       <div className="card__footer">
         <a className="button button--primary button--block" href="/sdks/quickstart/javascript">
-          Under development - coming soon
+          Get Started with JavaScript
         </a>
       </div>
     </div>