docs: getting started guide for Blaxel TODO demo

[maximilien]

Create a developer-facing getting started guide for the Blaxel TODO demo.

Scope

• project purpose
• prerequisites
• local startup steps
• health check
• basic usage verification
• common setup pitfalls

Acceptance criteria

• steps are ordered and testable
• assumptions are labeled
• guide is written in Markdown
• draft is ready for reviewer handoff

Notes

This repo currently appears to have a minimal README, so keep the guide practical and avoid inventing setup details.