Add link to Build Flag conditional page

[michael-hawker]

There's also the ability to use conditionals on build flags as called out here: https://docs.microsoft.com/en-us/windows/apps/windows-app-sdk/migrate-to-windows-app-sdk/misc-info

Would be useful to at least link to this other page from this document as this is the easier one that comes up when searching on bing.

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Document Details

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• ID: 3271c1ba-afb1-a77a-1dfc-f441f18dbf3f
• Version Independent ID: 8f0d1919-07c2-f12e-c3c5-e5a9f164d52e
• Content: Conditional XAML - UWP applications
• Content Source: windows-apps-src/debug-test-perf/conditional-xaml.md
• Product: uwp
• Technology: deployment
• GitHub Login: @Karl-Bridge-Microsoft
• Microsoft Alias: kbridge