Consolidate and expand section on design decisions and deviations

[mborland]

Closes: #1103

[cppalliance-bot]

An automated preview of the documentation is available at https://1104.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.

2025-10-10 07:30:40 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.4%. Comparing base (5446da0) to head (8295817).
⚠️ Report is 18 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1104     +/-   ##
=========================================
+ Coverage     99.4%   99.4%   +0.1%     
=========================================
  Files          245     245             
  Lines        16770   16770             
  Branches      1812    1812             
=========================================
+ Hits         16663   16664      +1     
+ Misses         107     106      -1     

see 1 file with indirect coverage changes

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