Update installation instructions to init first (lastmile-ai#582)

Author: StreetLamb

docs/get-started/install.mdx

@@ -28,9 +28,10 @@ The first run downloads the package into uv's cache; subsequent runs are instant
 
 ## Add mcp-agent to your project
 
-Inside your project directory, add mcp-agent as a dependency. Using uv:
+Inside your project directory, initialize a new project and add mcp-agent as a dependency:
 
 ```bash
+uv init
 uv add mcp-agent
 ```