examples: add math example (#1461)

* examples: add math example

* docs: add math example

docs/examples/math/ValueNode.vue

@@ -0,0 +1,22 @@
+<script setup>
+import { Handle, Position, useVueFlow } from '@vue-flow/core'
+
+const props = defineProps(['id', 'data'])
+
+const { updateNodeData } = useVueFlow()
+
+function onChange(event) {
+  const value = Number.parseFloat(event.target.value)
+
+  if (!Number.isNaN(value)) {
+    updateNodeData(props.id, { value })
+  }
+}
+</script>
+
+<template>
+  <label :for="`${id}-input`"> </label>
+  <input :id="`${id}-input`" :value="data.value" type="number" class="nodrag" @change="onChange" />
+
+  <Handle type="source" :position="Position.Right" :connectable="false" />
+</template>