docs: add CONTRIBUTING guide and visual content plan

[annabelgray89]

Summary

• Add CONTRIBUTING.md with prerequisites (Rust >= 1.85, Go for conformance tests), build/test/lint commands, repository layout, coding standards, wire format stability policy, and submission guidelines
• Add docs/VISUAL-CONTENT.md with terminal recording and diagram suggestions for the README's worked examples, design docs, and wire format specs
• Link CONTRIBUTING.md from the README's "Learn more" section

No changes to existing documentation content -- the README's worked examples are left as-is.

Test plan

• Verify all links in README resolve correctly
• Verify CONTRIBUTING.md build commands match CI workflow
• Confirm no existing content was modified beyond the link addition

🤖 Generated with Claude Code

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: e96f1c36-013f-458c-9fe3-a69549f4d58d

📥 Commits

Reviewing files that changed from the base of the PR and between 28e01a4 and 3e54050.

📒 Files selected for processing (3)

• CONTRIBUTING.md
• README.md
• docs/VISUAL-CONTENT.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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