docs: add cross-platform docs checklist

[slegarraga]

Closes #9

What

• Adds a ten-minute, Git-only workflow for first-time documentation contributors.
• Covers cloning/updating, optional Markdown preview, relative link/image checks, one focused edit, privacy-safe diff review, and opening a pull request.
• Includes Windows PowerShell, macOS, and Linux notes where behavior genuinely differs.
• Explicitly states that main is the public product-and-community documentation home, not production source.
• Links the checklist from CONTRIBUTING.md.

Verification

• Followed the checklist against the current repository clone.
• Verified every referenced local file exists.
• Verified the documented relative link target resolves to ROADMAP.md.
• Confirmed no Node.js/npm/build/test commands are introduced because no runnable source exists on main.
• Checked that all examples are synthetic and the diff contains only documentation changes.