Enhanced the documentation for building Uno.UI with single target platforms by improving step 5 and adding comprehensive examples and build optimization guidance.

Changes Made

Enhanced Step 5 Clarity

Updated step 5 to explicitly reference the crosstargeting_override.props.sample file where target framework identifiers are documented, making it clearer where developers can find the complete list of available platforms.

Added Concrete Examples

Added practical examples showing how to configure UnoTargetFrameworkOverride for common development scenarios:

<!-- iOS development with .NET 9 -->
<UnoTargetFrameworkOverride>net9.0-ios</UnoTargetFrameworkOverride>

<!-- Windows development with .NET 9 -->
<UnoTargetFrameworkOverride>net9.0-windows10.0.19041.0</UnoTargetFrameworkOverride>

<!-- Android development with .NET 9 -->
<UnoTargetFrameworkOverride>net9.0-android</UnoTargetFrameworkOverride>

New Build Optimization Section

Added a new "Additional Build Optimization Properties" section documenting:

• AccelerateBuildsInVisualStudio: Explains how this property improves build performance using reference assemblies builds, particularly useful when iterating on code changes
• OptimizeImplicitlyTriggeredBuild: Documents how this property speeds up builds by disabling C# analyzers, with appropriate warnings about commenting it out before creating pull requests

All documentation additions are consistent with the explanations provided in the crosstargeting_override.props.sample file to ensure accuracy and maintain a single source of truth. Examples have been updated to use .NET 9, which is the currently supported version.

Fixes #16680.

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