message: docs(Button): remove card style
diff: <template>
  <demo-section>
  - <demo-block card :title="t('type')">
  + <demo-block :title="t('type')">
  <div class="demo-button-row">
  <van-button type="default">{{ t('default') }}</van-button>
  <van-button type="primary">{{ t('primary') }}</van-button>
  <van-button type="warning">{{ t('warning') }}</van-button>
  </demo-block>
  - <demo-block card :title="t('plain')">
  + <demo-block :title="t('plain')">
  <van-button plain type="primary" :text="t('plain')" />
  <van-button plain type="info" :text="t('plain')" />
  </demo-block>
  - <demo-block card :title="t('hairline')">
  + <demo-block :title="t('hairline')">
  <van-button plain hairline type="primary" :text="t('hairlineButton')" />
  <van-button plain hairline type="info" :text="t('hairlineButton')" />
  </demo-block>
  - <demo-block card :title="t('disabled')">
  + <demo-block :title="t('disabled')">
  <van-button disabled type="primary" :text="t('disabled')" />
  <van-button disabled type="info" :text="t('disabled')" />
  </demo-block>
  - <demo-block card :title="t('loadingStatus')">
  + <demo-block :title="t('loadingStatus')">
  <van-button loading type="primary" />
  <van-button loading type="primary" loading-type="spinner" />
  <van-button loading :loading-text="t('loadingText')" type="info" />
  </demo-block>
  - <demo-block card :title="t('shape')">
  + <demo-block :title="t('shape')">
  <van-button type="primary" square :text="t('square')" />
  <van-button type="info" round :text="t('round')" />
  </demo-block>
  - <demo-block card :title="t('icon')">
  + <demo-block :title="t('icon')">
  <van-button type="primary" icon="star-o" />
  <van-button type="primary" icon="star-o" :text="t('button')" />
  <van-button
  />
  </demo-block>
  - <demo-block card :title="t('size')">
  + <demo-block :title="t('size')">
  <van-button type="primary" size="large">{{ t('large') }}</van-button>
  <van-button type="primary" size="normal">{{ t('normal') }}</van-button>
  <van-button type="primary" size="small">{{ t('small') }}</van-button>
  <van-button type="primary" size="mini">{{ t('mini') }}</van-button>
  </demo-block>
  - <demo-block card :title="t('blockElement')">
  + <demo-block :title="t('blockElement')">
  <van-button type="primary" block>{{ t('blockElement') }}</van-button>
  </demo-block>
  - <demo-block card v-if="!isWeapp" :title="t('router')">
  + <demo-block v-if="!isWeapp" :title="t('router')">
  <van-button
  :text="t('urlRoute')"
  type="primary"
  <van-button :text="t('vueRoute')" type="primary" to="index" />
  </demo-block>
  - <demo-block card :title="t('customColor')">
  + <demo-block :title="t('customColor')">
  <van-button color="#7232dd" :text="t('pure')" />
  <van-button plain color="#7232dd" :text="t('pure')" />
  <van-button
  