message: docs: update site subtitle
diff: <div class="van-doc-header__top">
  <a class="van-doc-header__logo">
  <img :src="config.logo" />
  - <span>{{ config.title }}</span>
  + <span class="van-doc-header__title">{{ config.title }}</span>
  + <span v-if="config.subtitle" class="van-doc-header__subtitle">
  + {{ config.subtitle }}
  + </span>
  </a>
  <ul class="van-doc-header__top-nav">
  @@ -241,21 +244,27 @@ export default {
  &__logo {
  display: block;
  - img,
  - span {
  - display: inline-block;
  - vertical-align: middle;
  - }
  -
  img {
  + display: inline-block;
  width: 28px;
  margin-right: 12px;
  + vertical-align: middle;
  + }
  }
  - span {
  + &__title {
  + display: inline-block;
  color: #fff;
  font-size: 22px;
  + vertical-align: middle;
  }
  +
  + &__subtitle {
  + display: inline-block;
  + color: #999;
  + margin-left: 8px;
  + vertical-align: -4px;
  + font-size: 13px;
  }
  &__link {
  