fix(nav): add baseUrl to Start Free button href

The HTML navbar item was using a raw href without the /robolearn baseUrl,
causing 404s when clicked on GitHub Pages deployment.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: mjunaidca

robolearn-interface/docusaurus.config.ts

@@ -289,7 +289,7 @@ const config: Config = {
         {
           type: "html",
           position: "right",
-          value: '<a href="/docs/preface-agent-native" class="navbar__cta-button">Start Free</a>',
+          value: '<a href="/robolearn/docs/preface-agent-native" class="navbar__cta-button">Start Free</a>',
         },
       ],
     },