Add api_endpoint config

Author: Folyd

docs/reference/configuration/ai.md

@@ -5,13 +5,19 @@
 ```toml
 [ai.openai]
 api_key = "YOUR_API_KEY"
+# optional, default is the official OpenAI API endpoint
+api_endpoint = "YOUR_API_ENDPOINT"
 model = "gpt-4"
 
 [ai.deepseek]
 api_key = "YOUR_API_KEY"
+# optional, default is the official DeepSeek API endpoint
+api_endpoint = "YOUR_API_ENDPOINT"
 model = "deepseek-chat"
 
 [ai.anthropic]
 api_key = "YOUR_API_KEY"
+# optional, default is the official Anthropic API endpoint
+api_endpoint = "YOUR_API_ENDPOINT"
 model = "claude-3-5-sonnet-latest"
 ```