docs: Add documentation for b3

[lusu007]

📝 Description

We are building b3, a small and lightweight web UI for building servers.
It provides authenticated uploads and management of schematics, brush heightmaps, and simple world uploads, with a strong focus on simplicity and builder-friendly workflows.

📖 Documentation Type

Missing documentation

📄 Current Documentation

None

✅ Expected Documentation

Document all features implement in b3

📍 Documentation Location

No response

📋 Additional Context

No response