examples: add math example (#1461)

* examples: add math example

* docs: add math example

docs/examples/math/OperatorNode.vue

@@ -0,0 +1,27 @@
+<script setup>
+import { Handle, Position, useVueFlow } from '@vue-flow/core'
+import Icon from './Icon.vue'
+
+const props = defineProps(['id', 'data'])
+
+const operators = ['+', '-', '*', '/']
+
+const { updateNodeData } = useVueFlow()
+</script>
+
+<template>
+  <div class="buttons nodrag">
+    <button
+      v-for="operator of operators"
+      :key="`${id}-${operator}-operator`"
+      :class="{ selected: data.operator === operator }"
+      @click="updateNodeData(props.id, { operator })"
+    >
+      <Icon :name="operator" />
+    </button>
+  </div>
+
+  <Handle type="source" :position="Position.Right" :connectable="false" />
+  <Handle id="target-a" type="target" :position="Position.Left" :connectable="false" />
+  <Handle id="target-b" type="target" :position="Position.Left" :connectable="false" />
+</template>