docs: split site with starlight-sidebar-topics

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 📝 Documentation: Split docs site into 'CLI' and 'Engine' #123
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Shoutout to https://starlight-sidebar-topics.netlify.app.

💝

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Name	Status	Preview	Comments	Updated (UTC)
create	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jan 14, 2025 6:31pm

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.51%. Comparing base (e921360) to head (78fdabe).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #131   +/-   ##
=======================================
  Coverage   98.51%   98.51%           
=======================================
  Files         179      179           
  Lines        6520     6520           
  Branches      961      961           
=======================================
  Hits         6423     6423           
  Misses         94       94           
  Partials        3        3           

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