In Google Docs you can create a drawing simply. "Create New" > "Drawing"
As a Drawing or Diagram cannot be Inserted from Google Sites directly.
You must embed the HTML code created when you publish the drawing to the Web "Share" Tab > "Publish to the Web"
The Drawing or Diagram will update automatically (Test by pressing F5 or Refresh the page), there is no need to publish it again.
The Google Drawing can be Embeded by adding the embed code to your html pages.