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 97.75%. Comparing base (1dd0704) to head (301626d).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5541      +/-   ##
==========================================
- Coverage   97.75%   97.75%   -0.01%     
==========================================
  Files         218      218              
  Lines       49498    49498              
==========================================
- Hits        48389    48388       -1     
- Misses       1109     1110       +1     

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