docs(plugin-google-analytics): add plugin reference

Author: meteorlxy

Diff for: docs/reference/plugin/google-analytics.md

@@ -1,3 +1,42 @@
 # google-analytics
 
-> TODO
+> @vuepress/plugin-google-analytics
+
+Integrate [Google Analytics](https://analytics.google.com/) into VuePress.
+
+This plugin will import [gtag.js](https://developers.google.com/analytics/devguides/collection/gtagjs) for [Google Analytics 4](https://support.google.com/analytics/answer/10089681).
+
+## Reporting Events
+
+Google Analytics will [automatically collect some events](https://support.google.com/analytics/answer/9234069), such as `page_view`, `first_visit`, etc.
+
+So if you only want to collect some basic data of your site, you don't need to do anything else except setting the [Measurement ID](#id) correctly.
+
+After using this plugin, the global `gtag()` function is available on the `window` object, and you can use it for [custom events reporting](https://developers.google.com/analytics/devguides/collection/ga4/events).
+
+## Options
+
+### id
+
+- Type: `string`
+
+- Details:
+
+  The Measurement ID of Google Analytics 4, which should start with `'G-'`.
+
+  You can follow the instructions [here](https://support.google.com/analytics/answer/9539598) to find your Measurement ID. Notice the difference between Google Analytics 4 Measurement ID (i.e. "G-" ID) and Universal Analytics Tracking ID (i.e. "UA-" ID).
+
+- Example:
+
+```js
+module.exports = {
+  plugins: [
+    [
+      '@vuepress/plugin-google-analytics',
+      {
+        id: 'G-XXXXXXXXXX',
+      },
+    ],
+  ],
+}
+```

Diff for: docs/zh/reference/plugin/google-analytics.md

@@ -1,3 +1,42 @@
 # google-analytics
 
-> TODO
+> @vuepress/plugin-google-analytics
+
+将 [Google Analytics](https://analytics.google.com/) 集成到 VuePress 中。
+
+该插件会通过引入 [gtag.js](https://developers.google.com/analytics/devguides/collection/gtagjs) 来启用 [Google Analytics 4](https://support.google.com/analytics/answer/10089681) 。
+
+## 上报事件
+
+Google Analytics 会 [自动收集部分事件](https://support.google.com/analytics/answer/9234069) ，比如 `page_view`, `first_visit` 等。
+
+因此，如果你只是想收集站点的一些基础数据，你只需要正确设置 [Measurement ID](#id) ，不需要再额外做其他事情。
+
+在引入该插件之后，一个全局的 `gtag()` 函数会被挂载到 `window` 对象上，你可以使用它进行 [自定义事件的上报](https://developers.google.com/analytics/devguides/collection/ga4/events) 。
+
+## 配置项
+
+### id
+
+- 类型： `string`
+
+- 详情：
+
+  Google Analytics 4 的 Measurement ID ，应以 `'G-'` 开头。
+
+  你可以通过 [这里](https://support.google.com/analytics/answer/9539598) 的指引来找到你的 Measurement ID 。注意区分 Google Analytics 4 的 Measurement ID （即 "G-" 开头的 ID） 和 Universal Analytics 的 Tracking ID （即 "UA-" 开头的 ID）。
+
+- 示例：
+
+```js
+module.exports = {
+  plugins: [
+    [
+      '@vuepress/plugin-google-analytics',
+      {
+        id: 'G-XXXXXXXXXX',
+      },
+    ],
+  ],
+}
+```