docs: add dagre layout example

docs/examples/layout/App.vue

@@ -0,0 +1,70 @@
+<script setup>
+import dagre from 'dagre'
+import { nextTick, ref } from 'vue'
+import { Panel, Position, VueFlow, useVueFlow } from '@vue-flow/core'
+
+import { initialEdges, initialNodes } from './initial-elements.js'
+
+const nodes = ref(initialNodes)
+
+const edges = ref(initialEdges)
+
+const { findNode, fitView } = useVueFlow()
+
+function handleLayout(direction) {
+  // we create a new graph instance, in case some nodes/edges were removed, otherwise dagre would act as if they were still there
+  const dagreGraph = new dagre.graphlib.Graph()
+
+  dagreGraph.setDefaultEdgeLabel(() => ({}))
+
+  const isHorizontal = direction === 'LR'
+  dagreGraph.setGraph({ rankdir: direction })
+
+  for (const node of nodes.value) {
+    // if you need width+height of nodes for your layout, you can use the dimensions property of the internal node (`GraphNode` type)
+    const graphNode = findNode(node.id)
+
+    dagreGraph.setNode(node.id, { width: graphNode.dimensions.width || 150, height: graphNode.dimensions.height || 50 })
+  }
+
+  for (const edge of edges.value) {
+    dagreGraph.setEdge(edge.source, edge.target)
+  }
+
+  dagre.layout(dagreGraph)
+
+  // set nodes with updated positions
+  nodes.value = nodes.value.map((node) => {
+    const nodeWithPosition = dagreGraph.node(node.id)
+
+    return {
+      ...node,
+      targetPosition: isHorizontal ? Position.Left : Position.Top,
+      sourcePosition: isHorizontal ? Position.Right : Position.Bottom,
+      position: { x: nodeWithPosition.x, y: nodeWithPosition.y },
+    }
+  })
+
+  nextTick(() => {
+    fitView()
+  })
+}
+</script>
+
+<template>
+  <div class="layoutflow">
+    <VueFlow :nodes="nodes" :edges="edges" @nodes-initialized="handleLayout('TB')">
+      <Panel style="display: flex; gap: 1rem" position="top-right">
+        <button @click="handleLayout('TB')">vertical layout</button>
+        <button @click="handleLayout('LR')">horizontal layout</button>
+      </Panel>
+    </VueFlow>
+  </div>
+</template>
+
+<style>
+.layoutflow {
+  height: 100%;
+  width: 100%;
+}
+</style>