docs: restructure docs into diataxis (#121)

docs/tutorials/install-and-onboard.md

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+# Install & onboard
+
+You’ll install Takopi, connect it to Telegram, and generate a working `takopi.toml`.
+
+## Prerequisites
+
+- A Telegram account
+- Python 3.14+ and `uv`
+- At least one supported engine CLI on your `PATH` (`codex`, `claude`, `opencode`, or `pi`)
+
+## 1) Install Takopi
+
+```sh
+uv tool install -U takopi
+```
+
+## 2) Run onboarding
+
+Start Takopi:
+
+```sh
+takopi
+```
+
+If you want to re-run onboarding later:
+
+```sh
+takopi --onboard
+```
+
+The wizard walks you through:
+
+1. Creating a bot token via [@BotFather](https://t.me/BotFather)
+2. Capturing your `chat_id` (it listens for a message from you)
+3. Choosing a default engine
+
+Your configuration lives at `~/.takopi/takopi.toml`.
+
+## 3) Verify minimal config
+
+After onboarding you should have something like:
+
+```toml
+default_engine = "codex"
+transport = "telegram"
+
+[transports.telegram]
+bot_token = "123456789:ABCdefGHIjklMNOpqrsTUVwxyz"
+chat_id = 123456789
+```
+
+## Next
+
+- [First run](first-run.md)
+- [Config reference](../reference/config.md)
+