message: docs: add vant use install guide
diff: Vant provide some built-in composition APIs, you can directly use these APIs for development.
  +### Install
  +
  +Although `@vant/use` is already included in Vant's dependencies, it is still recommended to install this package explicitly:
  +
  +```shell
  +# with npm
  +npm i @vant/use
  +
  +# with yarn
  +yarn add @vant/use
  +
  +# with pnpm
  +pnpm add @vant/use
  +```
  +
  ### Demo
  ```js
  