Document device-independent units and their relationship to platform-specific units

[Copilot]

This PR adds comprehensive documentation for device-independent units in .NET MAUI, addressing a key gap in the documentation where this concept was mentioned frequently but never clearly defined.

What's Added

New Documentation File

Created /docs/user-interface/device-independent-units.md that provides:

• Clear definition: Device-independent units (DIUs) represent approximately 1/160th of an inch of physical screen space
• Platform mapping: Explains the 1:1 relationship to native unit systems:
  • iOS: 1 DIU = 1 point
  • Android: 1 DIU = 1 density-independent pixel (dp)
  • Windows: 1 DIU = 1 device-independent pixel
  • macOS: 1 DIU = 1 point
• Practical examples: Code samples showing usage in layouts, fonts, animations, and responsive design
• Best practices: Guidelines for touch targets, common spacing values, and avoiding manual conversions

Enhanced Cross-References

Updated existing files that mention device-independent units to link to the new comprehensive documentation:

• fonts.md - FontSize property and font icon sizing
• align-position.md - Margin and padding measurements
• fundamentals/gestures/swipe.md - SwipeGestureRecognizer threshold
• fundamentals/triggers.md - AdaptiveTrigger window measurements
• user-interface/includes/basic-animation-dotnet10.md - Animation translation values

Navigation Updates

Added the new documentation to the Table of Contents under User Interface > Controls, positioned logically after "Align and position controls".

Why This Matters

The documentation clarifies that device-independent units in .NET MAUI are not just a naming difference from platform-specific units, but rather a unified abstraction that automatically handles density scaling across platforms. This provides developers with the understanding they need to create consistent UI layouts that work properly across iOS, Android, Windows, and macOS devices.

Fixes #1042.

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[learn-build-service-prod]

Learn Build status updates of commit 507100a:

✅ Validation status: passed

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[learn-build-service-prod]

Learn Build status updates of commit ea28a1a:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/fundamentals/gestures/swipe.md	✅Succeeded	View (>=net-maui-8.0)
docs/fundamentals/triggers.md	✅Succeeded	View (>=net-maui-8.0)
docs/TOC.yml	✅Succeeded	View (>=net-maui-8.0)
docs/user-interface/align-position.md	✅Succeeded	View (>=net-maui-8.0)
docs/user-interface/device-independent-units.md	✅Succeeded	View (>=net-maui-8.0)
docs/user-interface/fonts.md	✅Succeeded	View (>=net-maui-8.0)
docs/user-interface/includes/basic-animation-dotnet10.md	✅Succeeded	View (>=net-maui-8.0)

For more details, please refer to the build report.