Improve Sandbox Environment Docs

[ryw]

Summary

Comprehensive update to the sandbox environment documentation, focusing on:

• Simplified tool listing with Nix flake-based versioning
• Added details on sandbox types, workspace layout, and environment characteristics
• Introduced new sections on dev containers, environment variables, and best practices

Key changes include:

• Replaced detailed tool version table with more dynamic description
• Added information about QEMU and Docker sandbox capabilities
• Expanded on Nix development shell and dev container support
• Included new environment variable and configuration details

The documentation now provides a more flexible and informative overview of the Tembo Sandbox environment.

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