docs(models-http-api): add llamafile example (#3403)

* docs(models-http-api): add llamafile example

* docs(models-http-api): add completion support for llamafile

website/docs/references/models-http-api/llamafile.md

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+# llamafile
+
+[llamafile](https://github.com/Mozilla-Ocho/llamafile)
+is a Mozilla Builders project that allows you to distribute and run LLMs with a single file.
+
+llamafile embeds a llama.cpp server and provides an OpenAI API-compatible chat-completions endpoint,
+allowing us to use the `openai/chat`, `llama.cpp/completion`, and `llama.cpp/embedding` types.
+
+By default, llamafile uses port `8080`, which is also used by Tabby.
+Therefore, it is recommended to run llamafile with the `--port` option to serve on a different port, such as `8081`.
+
+For embeddings, the embedding endpoint is no longer supported in the standard llamafile server,
+so you need to run llamafile with the `--embedding` and `--port` options.
+
+Below is an example configuration:
+
+```toml title="~/.tabby/config.toml"
+# Chat model
+[model.chat.http]
+kind = "openai/chat"
+model_name = "your_model"
+api_endpoint = "http://localhost:8081/v1"
+api_key = ""
+
+# Completion model
+[model.completion.http]
+kind = "llama.cpp/completion"
+model_name = "your_model"
+api_endpoint = "http://localhost:8081"
+api_key = "secret-api-key"
+prompt_template = "<|fim_prefix|>{prefix}<|fim_suffix|>{suffix}<|fim_middle|>" # Example prompt template for the Qwen2.5 Coder model series.
+
+# Embedding model
+[model.embedding.http]
+kind = "llama.cpp/embedding"
+model_name = "your_model"
+api_endpoint = "http://localhost:8082"
+api_key = ""
+```