docs: translate docs of composables (#10561)

packages/vant/docs/markdown/use-relation.en-US.md

@@ -0,0 +1,80 @@
+# useRelation
+
+### Intro
+
+Establish the association relationship between parent and child components, perform data communication and method invocation, based on `provide` and `inject` implementation.
+
+## Usage
+
+### Basic Usage
+
+Use `useChildren` in parent to associate child components:
+
+```js
+import { ref } from 'vue';
+import { useChildren } from '@vant/use';
+
+const RELATION_KEY = Symbol('my-relation');
+
+export default {
+  setup() {
+    const { linkChildren } = useChildren(RELATION_KEY);
+
+    const count = ref(0);
+    const add = () => {
+      count.value++;
+    };
+
+    // provide data and methods to children
+    linkChildren({ add, count });
+  },
+};
+```
+
+Use `useParent` in child component to get the data and methods provided by parent.
+
+```js
+import { useParent } from '@vant/use';
+
+export default {
+  setup() {
+    const { parent } = useParent(RELATION_KEY);
+
+    // use data and methods provided by parent
+    if (parent) {
+      parent.add();
+      console.log(parent.count.value); // -> 1
+    }
+  },
+};
+```
+
+## API
+
+### Type Declarations
+
+```ts
+function useParent<T>(key: string | symbol): {
+  parent?: T;
+  index?: Ref<number>;
+};
+
+function useChildren(key: string | symbol): {
+  children: ComponentPublicInstance[];
+  linkChildren: (value: any) => void;
+};
+```
+
+### Return Value of useParent
+
+| Name | Description | Type |
+| --- | --- | --- |
+| parent | Data and methods provided by parent | _any_ |
+| index | Index position of the current component in all child of the parent component | _Ref\<number>_ |
+
+### Return Value of useChildren
+
+| Name | Description | Type |
+| --- | --- | --- |
+| children | Component list of children | _ComponentPublicInstance[]_ |
+| linkChildren | Function to provide values to child | _(value: any) => void_ |