docs: add local setup guide and ignore mdbook build output

[aaniya22]

What

Adds a local development setup guide to CONTRIBUTING.md, plus a small drive-by fix to .gitignore.

Why

New contributors need a verified, working set of steps to get a local dev environment running. Closes #283 (double-check this issue number before posting).

How

• Every command in the guide was run against a fresh checkout before being documented — the setup commands and mdbook build docs all produce clean, real output.
• While verifying the mdbook preview instructions, mdbook build generated a docs/book/ output directory and some auto-generated stub chapter files (blank placeholders for SUMMARY.md entries with no content yet). These were removed from the working tree and are not part of this PR.
• Added docs/book/ to .gitignore so future contributors following this guide don't accidentally commit build output.

Checklist

• cargo test --workspace passes
• cargo clippy --workspace -- -D warnings passes
• cargo fmt --all applied
• CHANGELOG.md updated (if user-facing change) — not applicable, docs-only change
• Benchmarks added or updated (if performance-sensitive path) — not applicable

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Review details

⚙️ Run configuration

Configuration used: defaults

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Run ID: 9c142bbb-b80f-4422-88c6-766cf7c34d5c

📥 Commits

Reviewing files that changed from the base of the PR and between 3c07775 and d7908ff.

📒 Files selected for processing (2)

• .gitignore
• CONTRIBUTING.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[github-actions]

PR Check Summary

Item	Value
Branch	docs/local-setup-guide-283
Changed crates	none
Files changed	2

CI will run: build, test, clippy, fmt, cargo-deny, DCO, semver.
Reviewer assigned from CODEOWNERS.