docs(design): clean up tree docs

[xelaint]

PR Checklist

• [x] Commit message follows our contributing guidelines
• [ ] Tests added/updated (for bug fixes/features)
• [ ] Documentation added/updated (for bug fixes/features)

PR Type

• [ ] Bug fix
• [ ] Feature
• [ ] Style update
• [ ] Refactor
• [ ] Test
• [ ] Build
• [ ] CI
• [x] Docs
• [ ] Performance
• [ ] Other (please describe)

Current behavior

Part of: #3771

New behavior

Breaking change?

• [ ] Yes
• [x] No

Additional context

[xelaint]

@damienwebdev I trimmed down the usage docs to be less technical. Let me know if anything I removed should be moved into the API docs.