📝 docs: add new configuration item of clerk singup (#4188)

Author: dohooo

docs/self-hosting/advanced/feature-flags.mdx

@@ -46,5 +46,6 @@ You can achieve various feature combinations using the above configuration synta
 | `market`                  | Enables the assistant market functionality.     | Enabled       |
 | `speech_to_text`          | Enables speech-to-text functionality.           | Enabled       |
 | `knowledge_base`          | Enables the knowledge base functionality.       | Enabled       |
+| `clerk_sign_up`           | Enables the Clerk SignUp functionality.         | Enabled       |
 
 You can always check the [featureFlags](https://github.com/lobehub/lobe-chat/blob/main/src/config/featureFlags/schema.ts) to get the latest list of feature flags.

docs/self-hosting/advanced/feature-flags.zh-CN.mdx

@@ -42,5 +42,6 @@ tags:
 | `market`                  | 启用助手市场功能。               | 开启   |
 | `speech_to_text`          | 启用语音转文本功能。             | 开启   |
 | `knowledge_base`          | 启用知识库功能。                 | 开启   |
+| `clerk_sign_up`          | 启用Clerk注册功能。                 | 开启   |
 
 你可以随时检查 [featureFlags](https://github.com/lobehub/lobe-chat/blob/main/src/config/featureFlags/schema.ts) 以获取最新的特性标志列表。