docs: minimal user guide for v0.1.0 release

[pierrick-fonquerne]

Ship enough documentation for an external early adopter to install, configure and run their first LightShuttle stack without asking questions.

Scope

• Quickstart in the README pointing to the tutorial.
• First tutorial under docs/tutorial/getting-started.md: postgres plus one Node.js container reading the env-injected URL.
• Anti-promises restated upfront.
• Cross-link to ROADMAP.md and the relevant milestone.

Acceptance criteria

• Tutorial executes end-to-end on a clean macOS, Linux and Windows environment with Docker installed.
• Reading time of the quickstart under five minutes.
• No assumed Rust knowledge in the tutorial.
• Style consistent with the README tone.