message: Doc: update demo && nav
diff: <template>
  <div class="van-pay-order">
  <div class="van-pay-order__tip" v-show="tip || $slots.tip">
  - <slot name="tip">{{ tip }}</slot>
  + {{ tip }}<slot name="tip"></slot>
  </div>
  <div class="van-pay-order__bar">
  <div class="van-pay-order__price">
  