Document overriding built-in unit conversions

[angularsen]

Motivation

#1313 reported that custom conversion functions could not override built-in same-quantity conversions in v5. In v6, the supported model is to customize the quantity unit definitions before the converter is built, instead of adding runtime override precedence for built-in conversions.

Changes

• Document how to override built-in unit conversion factors through UnitsNetSetup.ConfigureDefaults() or an isolated custom UnitConverter.
• Add regression coverage for the original PressureUnit.InchOfWaterColumn scenario, verifying that UnitConverter, As(..., converter), and ToUnit(..., converter) all use the configured conversion factor.

Validation

• dotnet test UnitsNet.Tests\UnitsNet.Tests.csproj --filter "FullyQualifiedName~UnitConverterTest.ConvertValue_WithCustomBuiltInUnitDefinition_UsesConfiguredConversion|FullyQualifiedName~UnitConverterTest.GetConversionFunction_WithCustomUnitConversion|FullyQualifiedName~UnitConverterTest.TryGetConversionFunction_WithCustomUnitConversion"

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98%. Comparing base (af17f9b) to head (c3259a4).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1718   +/-   ##
======================================
  Coverage      98%     98%           
======================================
  Files         514     514           
  Lines       24141   24141           
======================================
  Hits        23748   23748           
  Misses        393     393           

Flag	Coverage Δ
net48	97% <ø> (ø)

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