docs: init readthedocs support (#783)

docs/en/custom_chat_template.md

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+# Custom Chat Template in SGLang Runtime
+
+By default, the server uses the chat template specified in the model tokenizer from Hugging Face. It should just work for most official models such as Llama-2/Llama-3.
+
+If needed, you can also override the chat template when launching the server:
+
+```
+python -m sglang.launch_server --model-path meta-llama/Llama-2-7b-chat-hf --port 30000 --chat-template llama-2
+```
+
+If the chat template you are looking for is missing, you are welcome to contribute it.
+Meanwhile, you can also temporarily register your chat template as follows:
+
+```json
+{
+  "name": "my_model",
+  "system": "<|im_start|>system",
+  "user": "<|im_start|>user",
+  "assistant": "<|im_start|>assistant",
+  "sep_style": "CHATML",
+  "sep": "<|im_end|>",
+  "stop_str": ["<|im_end|>", "<|im_start|>"]
+}
+```
+
+```
+python -m sglang.launch_server --model-path meta-llama/Llama-2-7b-chat-hf --port 30000 --chat-template ./my_model_template.json
+```