Refresh project documentation

[Bromeego]

Summary

• redesign every Markdown file with a clearer documentation hierarchy
• add a project-specific DESIGN.md based on the existing interface
• align setup, contribution, security, changelog, and pull-request guidance

Why

The documentation had accurate information but inconsistent structure and no durable interface design reference. This refresh gives users a faster path from setup to operation and gives future contributors explicit visual guardrails.

Impact

Documentation-only. Application behavior and deployment configuration are unchanged.

Verification

• 20 application tests pass
• all seven Markdown files have valid local links, images, and heading anchors
• git diff --check passes
• Compose validation was unavailable because Docker is not exposed inside the current WSL session