Add WPF breaking change documentation for MC3063 error with empty Grid definitions

[Copilot]

This PR adds documentation for a WPF breaking change introduced in .NET 10 Preview 5 where empty <Grid.ColumnDefinitions> or <Grid.RowDefinitions> declarations now cause MC3063 compilation errors.

Changes Made

• Created docs/core/compatibility/wpf/10.0/empty-grid-definitions.md - Complete breaking change documentation with examples and migration guidance
• Updated docs/core/compatibility/10.0.md - Added entry to the WPF section
• Updated docs/core/compatibility/toc.yml - Added navigation entry for the new breaking change

Breaking Change Details

Previously, WPF applications could compile successfully with empty Grid definitions like this:

<Grid>
  <Grid.ColumnDefinitions>
  </Grid.ColumnDefinitions>
  <Grid.RowDefinitions>
  </Grid.RowDefinitions>
  <TextBlock Text="Hello World" />
</Grid>

Starting with .NET 10 Preview 5, this now fails with:

error MC3063: Property 'ColumnDefinitions' does not have a value.
error MC3063: Property 'RowDefinitions' does not have a value.

The documentation provides two clear migration paths:

1. Remove the empty declarations entirely
2. Add actual <ColumnDefinition /> or <RowDefinition /> elements

This change is a consequence of implementing Grid XAML Shorthand Syntax support, which requires stricter parsing of property declarations.

Fixes #47743.

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Internal previews

📄 File	🔗 Preview link
docs/core/compatibility/10.0.md	Breaking changes in .NET 10
docs/core/compatibility/toc.yml	docs/core/compatibility/toc
docs/core/compatibility/wpf/10.0/empty-grid-definitions.md	Empty ColumnDefinitions and RowDefinitions are disallowed