Update sandbox terminology

[tembo]

Summary

Updated sandbox environment documentation to reflect new naming conventions:

• Renamed Standard sandbox to Small
• Renamed Large VM sandbox to Large Full VM
• Added clarification that Small sandbox cannot run Docker-in-Docker
• Added new section documenting that multiple sandboxes can run concurrently with no limits
• Updated resource allocation table to show Large Full VM uses 4 cores/8GB (not 8 cores/32GB)
• Updated all references throughout the document for consistency

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