[Docs] Codex/GitHub project doc

[learncold]

Summary

• Github Project를 설명하는 mark down 파일을 추가합니다.

Related Issue

• Closes Task-Add GitHub Project notes #42

Area

• Engine
• Domain
• Application
• Docs
• Build
• Analysis
• Chore

Architecture Check

• I kept the dependency direction application -> domain -> engine.
• I did not add Qt UI code to src/domain.
• I did not add domain or application dependencies to src/engine.
• I used src/ as the include root.

Verification

• cmake --preset windows-debug
• cmake --build --preset build-debug
• ctest --preset test-debug
• Not run (reason below)

Risks / Follow-up

• None.