docs: fix basic-host setup instructions in quickstart (modelcontextprotocol#398)

* docs: fix basic-host setup instructions in quickstart

Add missing root-level npm install and build steps before running
the basic-host example. Without these steps, the example fails because
it imports from dist/src/ which doesn't exist until the root package
is built.

Fixes setup failure when following quickstart tutorial.

* Update quickstart.md

Co-authored-by: Jonathan Hefner <jonathan@hefner.pro>

---------

Co-authored-by: Jonathan Hefner <jonathan@hefner.pro>

Author: rafa-thayto

docs/quickstart.md

@@ -447,8 +447,9 @@ MCP server listening on http://localhost:3001/mcp
 
 ```bash
 git clone https://github.com/modelcontextprotocol/ext-apps.git
-cd ext-apps/examples/basic-host
+cd ext-apps
 npm install
+cd examples/basic-host
 npm start
 ```