Rewrite Quickstart guide with type-checked code examples

Rewrites the Quickstart tutorial to provide a more focused, step-by-step
introduction to building MCP Apps. The guide now uses synced code fences
that pull from `examples/quickstart/`, ensuring all code shown in the
documentation is type-checked and tested.

Key improvements:
- Clearer project setup instructions with explicit commands
- Streamlined explanations of the tool + resource registration pattern
- Added screenshot showing the completed app
- E2E test coverage validates the example works end-to-end

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

examples/basic-host/src/index.module.css

@@ -145,11 +145,11 @@
 }
 
 .appIframePanel {
-  min-height: 200px;
+  min-height: 100px;
 
   iframe {
     width: 100%;
-    height: 600px;
+    height: 400px;
     box-sizing: border-box;
     border: 3px dashed var(--color-border);
     border-radius: 4px;