feat(tools): add tool to check for unintentional duplicate documentation

[suzmue]

This change introduces the doc-dup-checker tool in the tools directory. The tool analyzes Rustdoc JSON to find instances where both a re-export and its target have documentation, which rustdoc concatenates in the generated HTML.

The tool supports workspace-wide scanning of handwritten, published crates. It respects intentional paragraph breaks in re-export documentation as exceptions.

This does not yet add the tool to CI.

For #5481

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.76%. Comparing base (b1d4cb3) to head (1acea9c).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5541   +/-   ##
=======================================
  Coverage   97.76%   97.76%           
=======================================
  Files         219      219           
  Lines       50665    50665           
=======================================
  Hits        49531    49531           
  Misses       1134     1134           

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