misc: Add architecture documentation for XS-GEM5[skip ci]

[jensen-yan]

Change-Id: I9e76b21e7abafc40c45bc8d5847440ad04218126

Summary by CodeRabbit

• Documentation
  • Added comprehensive architecture documentation detailing repository structure, CPU pipeline orchestration, the decoupled frontend design, configuration-driven timing behavior, subsystem communication patterns, memory-side timing details, and code navigation guidance.
  • Updated README with reference to the new architecture documentation to improve project navigation and provide contributors with a high-level overview of the system.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 027aa1ee-b70a-4bb6-a050-cd70f250913a

📥 Commits

Reviewing files that changed from the base of the PR and between 103c503 and 2ec35e1.

📒 Files selected for processing (2)

• ARCHITECTURE.md
• README.md

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📝 Walkthrough

Walkthrough

A new ARCHITECTURE.md document is added to provide a comprehensive, repository-specific map of XS-GEM5's architecture, detailing the O3 CPU pipeline, decoupled frontend, and configuration-driven behavior. README.md is updated with a reference to this new guide.

Changes

Cohort / File(s)	Summary
Architecture Documentation ARCHITECTURE.md, README.md	Added comprehensive ARCHITECTURE.md document covering pipeline stages, code map, O3 orchestration, frontend assumptions, and configuration details. Updated README.md with "Architecture Notes" section referencing the new guide.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

Poem

🐰 A map of the machine is drawn with care,
O3 pipelines dance through the digital air,
Decoupled frontend, branching so neat,
Architecture documented—our treasure complete! 🏗️✨

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch add-arch-md

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