Add example on read and write to file

[mborland]

Closes: #1265

[cppalliance-bot]

An automated preview of the documentation is available at https://1267.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.

2025-12-05 08:17:58 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.9%. Comparing base (754cdba) to head (54346a2).
⚠️ Report is 34 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1267     +/-   ##
=========================================
- Coverage     98.9%   98.9%   -0.0%     
=========================================
  Files          266     266             
  Lines        17698   17698             
  Branches      1970    1970             
=========================================
- Hits         17493   17492      -1     
- Misses         205     206      +1     

see 1 file with indirect coverage changes

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