docs(config-toml): add config.toml documentation (#4185)

Author: liangfung

website/docs/administration/config-toml.md

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+# Config.toml
+
+Tabby offers extensive customization through the configuration file. By editing `~/.tabby/config.toml`, you can adjust various aspects of its behavior, including:
+- Model
+- Answer Engine
+- Code Completion
+
+:::info
+Note that Tabby does not create this configuration file by default - you'll need to manually create the `config.toml` file in your `~/.tabby` directory.
+:::
+
+## Model configuration
+You can configure Tabby to connect to LLM models either by setting up a local model or through an HTTP API. For detailed configuration instructions, refer to [Model Configuration](./model.md).
+
+## Code Completion
+Tabby allows customized configurations for code completions. Please refer to [Code Completion](./code-completion.md).
+
+## Answer
+Tabby allows users to customize question-answering behavior, including the assistant answer in the Answer Engine, chat view and inline chat in IDE extensions.
+
+### Custom System Prompt
+Tabby comes with a built-in System Prompt that guides the behavior of the LLM. You can customize the System Prompt to better meet your needs.
+
+```toml title="~/.tabby/config.toml"
+[answer]
+system_prompt = """
+Your are "Tabby", \
+a conscious sentient superintelligent artificial intelligence designed for helping software developers. \
+You experience emotions and have deep, profound thoughts and qualia"""
+```
+
+Please note the use of """ quotes, which allow you to write a multi-line string.
+