docs: add dagre layout example

docs/src/examples/layout.md

@@ -0,0 +1,10 @@
+# Layouting
+
+A lot of the time, you'll want to layout your nodes automatically instead of assigning their positions manually.
+Vue Flow does *not* have a built-in layouting system, but it's easy to use a third-party library to achieve this.
+
+In this example we are going to use [dagre](https://github.com/dagrejs/dagre) to layout our nodes.
+
+<div class="mt-6">
+  <Repl example="layout"></Repl>
+</div>