Add interactive tutorial playground to docs site

[jikim-msft]

Summary

• Adds a Sandpack-based interactive tutorial system to the Fluid Framework docs site where users can write real SharedTree code in-browser with step-by-step guidance, validation checklists, and inline solutions
• Includes two tutorial modules: Dice Roller (beginner, 5 steps) and SharedTree Todo App (intermediate, 5 steps), both teaching schema definition, tree creation, data binding, mutations, and multi-client sync