docs(coding-agent): add quickstart guide

packages/coding-agent/docs/index.md

@@ -10,7 +10,7 @@ Install pi with npm:
 npm install -g @mariozechner/pi-coding-agent
 ```
 
-And run it:
+Then run it in a project directory:
 
 ```bash
 pi
@@ -18,11 +18,12 @@ pi
 
 Authenticate with `/login` for subscription providers, or set an API key such as `ANTHROPIC_API_KEY` before starting pi.
 
-Once you are signed in, you can ask pi about itself and it will answer you.  No
-need to read the docs yourself ;-)
+For the full first-run flow, see [Quickstart](quickstart.md).
 
 ## Start here
 
+- [Quickstart](quickstart.md) - install, authenticate, and run a first session.
+- [Using Pi](usage.md) - interactive mode, slash commands, context files, and CLI reference.
 - [Providers](providers.md) - subscription and API-key setup for built-in providers.
 - [Settings](settings.md) - global and project settings.
 - [Keybindings](keybindings.md) - default shortcuts and custom keybindings.