fix: allow README.md explainer to be a full section

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: README.md h2 sections between badges and Usage shouldn't be removed #2127
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Moves the explainer option into documentation.readme so it sits alongside additional.

🎁

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.81%. Comparing base (7cae646) to head (0d7e058).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2128   +/-   ##
=======================================
  Coverage   99.80%   99.81%           
=======================================
  Files         127      127           
  Lines        5246     5265   +19     
  Branches      371      375    +4     
=======================================
+ Hits         5236     5255   +19     
  Misses         10       10           

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[github-actions]

🎉 This is included in version v2.24.3 🎉

The release is available on:

• GitHub releases
• npm package (@latest dist-tag)

Cheers! 📦🚀