fix(marketplace): Update URLs to use /marketplace path and relative API endpoints

- Change API_BASE to relative '/api' for production
- Move marketplace to /marketplace instead of /marketplace/frontend
- Update MkDocs navigation
- Fix logo path in marketplace index

Author: unclecode

docs/md_v2/marketplace/admin/admin.js

@@ -1,5 +1,5 @@
 // Admin Dashboard - Smart & Powerful
-const API_BASE = 'http://localhost:8100/api';
+const API_BASE = '/api';
 
 class AdminDashboard {
     constructor() {