Fix display of bold in docs

[ianthomas23]

Fix the use of ansi_code::bold and ansi_code::reset in HTML docs. These now map across to <span> elements using CSS of font-weight: bold.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.85%. Comparing base (5b2e65e) to head (b1577c1).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #152      +/-   ##
==========================================
- Coverage   88.52%   86.85%   -1.67%     
==========================================
  Files          62       62              
  Lines        2884     2884              
  Branches      358      357       -1     
==========================================
- Hits         2553     2505      -48     
- Misses        331      379      +48     

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[ianthomas23]

Example of bold output at https://git2cpp--152.org.readthedocs.build/en/152/created/git2cpp/stash.html