docs: add useCustomFieldValue document (#9202)

docs/markdown/use-custom-field-value.en-US.md

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+# useCustomFieldValue
+
+### Intro
+
+Used to custom Field value.
+
+## Usage
+
+### Basic Usage
+
+If you want to custom Form items, you can insert your component into the `input` slot of the Field component, and call the `useCustomFieldValue` method inside your custom component.
+
+#### MyComponent
+
+```js
+// MyComponent.vue
+import { useCustomFieldValue } from '@vant/use';
+
+export default {
+  setup() {
+    useCustomFieldValue(() => 'Some value');
+  },
+};
+```
+
+#### Form
+
+```html
+<van-form>
+  <van-field name="my-field" label="Custom Field">
+    <template #input>
+      <my-component />
+    </template>
+  </van-field>
+</van-form>
+```
+
+## API
+
+### Type Declarations
+
+```ts
+function useCustomFieldValue(customValue: () => unknown): void;
+```
+
+### Params
+
+| Name        | Description                 | Type            | Default Value |
+| ----------- | --------------------------- | --------------- | ------------- |
+| customValue | Function to get field value | _() => unknown_ | -             |