docs: add docs for cascader

Change-Id: Id5f4d5287f18417e91ff3986f1ab48354ab5bba8

one/docs/demo/cascader/searchable.vue

@@ -0,0 +1,128 @@
+<template>
+<article>
+  <veui-cascader
+    v-model="value"
+    searchable
+    :options="options"
+  />
+</article>
+</template>
+
+<script>
+import { Cascader } from 'veui'
+
+const options = [
+  {
+    label: '浙江',
+    value: '浙江',
+    options: [
+      {
+        label: '威海',
+        value: '威海'
+      },
+      {
+        label: '滨州',
+        value: '滨州'
+      },
+      {
+        label: '临沂',
+        value: '临沂'
+      },
+      {
+        label: '东营',
+        value: '东营'
+      },
+      {
+        label: '济南',
+        value: '济南'
+      }
+    ]
+  },
+  {
+    label: '山东',
+    value: '山东',
+    options: [
+      {
+        label: '菏泽',
+        value: '菏泽',
+        options: [
+          {
+            label: '菏泽1',
+            value: '菏泽1'
+          }
+        ]
+      },
+      {
+        label: '潍坊',
+        value: '潍坊',
+        options: [
+          {
+            label: '潍坊1',
+            value: '潍坊1'
+          }
+        ]
+      },
+      {
+        label: '泰山',
+        value: '泰山'
+      },
+      {
+        label: '烟台',
+        value: '烟台'
+      },
+      {
+        label: '华山',
+        value: '华山'
+      },
+      {
+        label: '衡山',
+        value: '衡山'
+      },
+      {
+        label: '嵩山',
+        value: '嵩山'
+      },
+      {
+        label: '恒山',
+        value: '恒山'
+      },
+      {
+        label: '大雪山',
+        value: '大雪山'
+      }
+    ]
+  },
+  {
+    label: '上海',
+    value: '上海',
+    disabled: true
+  },
+  {
+    label: '北京',
+    value: '北京'
+  },
+  {
+    label: '海外',
+    value: '海外',
+    disabled: true,
+    options: [
+      {
+        label: '日本',
+        value: '日本'
+      }
+    ]
+  }
+]
+
+export default {
+  components: {
+    'veui-cascader': Cascader
+  },
+  data () {
+    return {
+      value: null,
+      options
+    }
+  }
+}
+</script>