message: docs(NumberKeyboard): fix missing demo
diff: <template>
  <demo-section>
  + <demo-block :title="$t('default')">
  + <van-button
  + type="primary"
  + @touchstart.stop="keyboard = 'default'"
  + >
  + {{ $t('button1') }}
  + </van-button>
  +
  <van-number-keyboard
  :show="keyboard === 'default'"
  :close-button-text="$t('close')"
  extra-key="."
  + safe-area-inset-bottom
  @blur="keyboard = ''"
  @input="onInput"
  @delete="onDelete"
  />
  + </demo-block>
  +
  + <demo-block :title="$t('custom')">
  + <van-button
  + type="info"
  + @touchstart.stop="keyboard = 'custom'"
  + >
  + {{ $t('button2') }}
  + </van-button>
  <van-number-keyboard
  :show="keyboard === 'custom'"
  @input="onInput"
  @delete="onDelete"
  />
  + </demo-block>
  +
  + <demo-block :title="$t('bindValue')">
  + <van-field
  + readonly
  + clickable
  + :value="value"
  + :placeholder="$t('clickToInput')"
  + @touchstart.native.stop="keyboard = 'bindValue'"
  + />
  +
  + <van-number-keyboard
  + v-model="value"
  + :show="keyboard === 'bindValue'"
  + maxlength="6"
  + safe-area-inset-bottom
  + @blur="keyboard = ''"
  + />
  + </demo-block>
  </demo-section>
  </template>
  