docs: 完善文档

docs/WechatWork/Advanced_Extensions.md

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+## 如何扩展额外的 API？
+
+---
+
+如果有某些接口本库尚未支持，你可按照下面的示例自行扩展：
+
+```csharp
+/* 继承 WechatWorkRequest 实现自定义请求类 */
+public class MyFakeRequest : WechatWorkRequest
+{
+    [Newtonsoft.Json.JsonProperty("my_fake_props")]
+    [System.Text.Json.Serialization.JsonPropertyName("my_fake_props")]
+    public string MyFakeProps { get; set; }
+}
+
+/* 继承 WechatWorkResponse 实现自定义响应类 */
+public class MyFakeResponse : WechatWorkResponse
+{
+    [Newtonsoft.Json.JsonProperty("my_fake_props")]
+    [System.Text.Json.Serialization.JsonPropertyName("my_fake_props")]
+    public string MyFakeProps { get; set; }
+}
+
+/* 扩展 WechatWorkClient 方法 */
+public static class MyFakeClientExtensions
+{
+    public static async Task<MyFakeResponse> ExecuteMyFakeAsync(this WechatWorkClient client, MyFakeRequest request, CancellationToken cancellationToken = default)
+    {
+        if (client is null) throw new ArgumentNullException(nameof(client));
+        if (request is null) throw new ArgumentNullException(nameof(request));
+
+        IFlurlRequest flurlReq = client
+            .CreateRequest(request, HttpMethod.Post, "my-fake-url")
+            .SetQueryParam("access_token", request.AccessToken);
+
+        return await client.SendRequestWithJsonAsync<MyFakeResponse>(flurlReq, request, cancellationToken);
+    }
+}
+```
+
+同样的，你也可自行扩展回调通知事件模型：
+
+```csharp
+/* 继承 WechatWorkEvent 实现自定义的 JSON 格式的回调通知事件 */
+public class MyFakeEvent : WechatWorkEvent, WechatWorkEvent.Types.IJsonSerializable
+{
+    [Newtonsoft.Json.JsonProperty("my_fake_props")]
+    [System.Text.Json.Serialization.JsonPropertyName("my_fake_props")]
+    public string MyFakeProps { get; set; }
+}
+
+/* 继承 WechatWorkEvent 实现自定义的 XML 格式的回调通知事件 */
+public class MyFakeEvent : WechatWorkEvent, WechatWorkEvent.Types.IXmlSerializable
+{
+    [System.Xml.Serialization.XmlElement("my_fake_props")]
+    public string MyFakeProps { get; set; }
+}
+```