docs(api): add delete proxy endpoint documentation

Add documentation for the new DELETE proxy endpoint including request/response formats, headers, and important notes about the operation.

docs/api.md

@@ -721,6 +721,69 @@ remote_port = 6000
 
 ---
 
+## Delete Proxy
+
+**Endpoint:** `/frpcAct/proxyMgr/delete`  
+**Method:** POST  
+**Content-Type:** application/json  
+**Auth Required:** Yes (token)  
+**Permission Level:** Admin
+
+Delete an existing proxy configuration from an frpc instance. The proxy configuration will be removed from the instance's config file.
+
+**Request Headers:**
+```
+X-Token: your_token
+X-Timestamp: 1704067200000
+```
+
+**Request Body:**
+```json
+{
+  "instanceID": "1",
+  "proxyName": "ssh_proxy"
+}
+```
+
+| Header | Type | Required | Description |
+|--------|------|----------|-------------|
+| X-Token | string | Yes | Authentication token |
+| X-Timestamp | int64 | Yes | Client timestamp in milliseconds |
+
+| Field | Type | Required | Description |
+|-------|------|----------|-------------|
+| instanceID | string | Yes | Instance ID (the ID of the frpc instance) |
+| proxyName | string | Yes | Name of the proxy to delete |
+
+**Response:**
+```json
+{
+  "success": true,
+  "message": "Proxy deleted successfully",
+  "data": {
+    "instanceName": "my_frpc",
+    "instanceID": 1,
+    "configPath": "./configs/superfrpc_user_my_frpc.toml",
+    "proxyName": "ssh_proxy"
+  }
+}
+```
+
+| Field | Type | Description |
+|-------|------|-------------|
+| instanceName | string | Name of the frpc instance |
+| instanceID | int | Instance ID |
+| configPath | string | Path to the configuration file |
+| proxyName | string | Name of the deleted proxy |
+
+> **Note:** 
+> - The proxy configuration is removed from the existing config file
+> - The instance must already exist before deleting a proxy
+> - This endpoint does not modify the database, only the config file
+> - The frpc service needs to be restarted for changes to take effect
+
+---
+
 ## List frpc Instances
 
 **Endpoint:** `/frpcAct/instanceMgr/list`