Update format docs and examples

[mborland]

Closes: #1214
Closes: #1201

Also changes the feature test macro for <format>

[cppalliance-bot]

An automated preview of the documentation is available at https://1222.decimal.prtest3.cppalliance.org/libs/decimal/doc/html/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2025-11-06 13:52:47 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.1%. Comparing base (0bc45a2) to head (e365f70).
⚠️ Report is 40 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1222     +/-   ##
=========================================
- Coverage     99.1%   99.1%   -0.0%     
=========================================
  Files          260     260             
  Lines        17158   17158             
  Branches      1935    1935             
=========================================
- Hits         16995   16994      -1     
- Misses         163     164      +1     

see 1 file with indirect coverage changes

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