docs: add project architecture section to README

[radhikatbbt]

What

Adds a new "Project Architecture" section to README.md, briefly explaining
the purpose of crates/, tests/, examples/, docs/, benches/, and
scripts/, plus a simple repository tree diagram.

Why

The README currently covers vision, goals, and tech stack, but gives new
contributors no overview of how the repo is structured, making it harder to
know where to start. Closes #282.

How

Inserted the new section between ## what's coming and ## status, matching
the existing README's lowercase header style and plain bullet formatting.
No code was changed — this is a documentation-only edit.

Checklist

• cargo test --workspace passes — N/A, no code changed
• cargo clippy --workspace -- -D warnings passes — N/A, no code changed
• cargo fmt --all applied — N/A (Markdown file, not applicable, unchanged)
• CHANGELOG.md updated (if user-facing change) — not applicable, internal docs addition only
• Benchmarks added or updated (if performance-sensitive path) — N/A, no performance-sensitive code touched

Summary by CodeRabbit

• Documentation
  • Added a new project architecture section to the README, outlining the repository layout and where to find key areas like library code, tests, examples, docs, benchmarks, and scripts.
  • Included a clearer directory-style view of the workspace structure.
  • Updated the license section formatting for improved readability.

[github-actions]

PR Check Summary

Item	Value
Branch	docs/project-architecture-section
Changed crates	none
Files changed	1

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

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 7f6e4c85-5f14-4618-b003-c6263e3a3ba8

📥 Commits

Reviewing files that changed from the base of the PR and between a814f86 and 6ad9d91.

📒 Files selected for processing (1)

• README.md

📝 Walkthrough

Walkthrough

The README was updated with a new "project architecture" section describing the Cargo workspace layout, including a bullet list of key directories and a directory tree. A trailing blank line was also added after the license text.

Changes

README documentation update

Layer / File(s)	Summary
Add project architecture section README.md	Adds a "project architecture" section describing the Cargo workspace layout, key directories, and a directory tree; adds a trailing blank line after the MIT license text.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Related issues: #282

Suggested labels: documentation

Suggested reviewers:

_{🐰 A rabbit hops through docs anew,
Mapping crates and tests in view,
A tree of folders, neat and clear,
One blank line added, license near,
Onboarding made easier too!}

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main README documentation change.
Linked Issues check	✅ Passed	The README adds a Project Architecture section, directory descriptions, and a tree diagram as requested.
Out of Scope Changes check	✅ Passed	The only changes are in README.md and stay within the requested documentation update.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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