fix: add most recent ballot hint to History tab.

[russhousley]

Fixes #9696.
Add a hint to the History tab for balloters that tells the revision of the document associated with the most recent ballot.

[codecov]

Codecov Report

❌ Patch coverage is 84.21053% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.54%. Comparing base (cbb0e2e) to head (bb7da11).
⚠️ Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
ietf/doc/utils.py	86.66%	2 Missing ⚠️
ietf/doc/views_doc.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9851      +/-   ##
==========================================
- Coverage   88.54%   88.54%   -0.01%     
==========================================
  Files         316      316              
  Lines       42265    42282      +17     
==========================================
+ Hits        37423    37437      +14     
- Misses       4842     4845       +3     

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[jennifer-richards]

Trivial nit suggested