docs: add headless mode guide (#8564)

docs/cli/index.md

@@ -8,6 +8,7 @@ Within Gemini CLI, `packages/cli` is the frontend for users to send and receive
 - **[Commands](./commands.md):** A reference for Gemini CLI commands (e.g., `/help`, `/tools`, `/theme`).
 - **[Configuration](./configuration.md):** A guide to tailoring Gemini CLI behavior using configuration files.
 - **[Enterprise](./enterprise.md):** A guide to enterprise configuration.
+- **[Headless Mode](./headless.md):** A comprehensive guide to using Gemini CLI programmatically for scripting and automation.
 - **[Token Caching](./token-caching.md):** Optimize API costs through token caching.
 - **[Themes](./themes.md)**: A guide to customizing the CLI's appearance with different themes.
 - **[Tutorials](tutorials.md)**: A tutorial showing how to use Gemini CLI to automate a development task.
@@ -22,24 +23,10 @@ The following example pipes a command to Gemini CLI from your terminal:
 echo "What is fine tuning?" | gemini
 ```
 
-Gemini CLI executes the command and prints the output to your terminal. Note that you can achieve the same behavior by using the `--prompt` or `-p` flag. For example:
+You can also use the `--prompt` or `-p` flag:
 
 ```bash
 gemini -p "What is fine tuning?"
 ```
 
-For non-interactive usage with structured output, use the `--output-format json` flag for scripting and automation.
-
-Get structured JSON output for scripting:
-
-```bash
-gemini -p "What is fine tuning?" --output-format json
-# Output:
-# {
-#   "response": "Fine tuning is...",
-#   "stats": {
-#     "models": { "gemini-2.5-flash": { "tokens": {"total": 45} } }
-#   },
-#   "error": null
-# }
-```
+For comprehensive documentation on headless usage, scripting, automation, and advanced examples, see the **[Headless Mode](./headless.md)** guide.