feat: Avalonia components in the browser for the docs previews

[Shewart]

What

• New examples/Avalonia.Preview: the Avalonia components compiled to WebAssembly, showing one preview per address (?c=button, &theme=dark|light). It compiles the demo's own component files, so it can't drift from them.
• The docs site supplies its own previews with -p:PreviewsDir=<folder>; the four in Previews/ are samples.
• The theme follows the hosting page (shelldocs-theme messages), and the app reports its content height.
• Not in the solution, so CI and the solution build don't need the WebAssembly workload.

Merge with a merge commit: the docs site's Dockerfile pins b3d98d4 from this branch until it moves to main.

Checked

• dotnet build -c Release: 0 warnings, with and without PreviewsDir.
• Published size: 16.4 MB uncompressed, about 4.5 MB with Brotli. Starts in about 2 s from an empty cache on localhost.
• 81 docs previews render in both themes in headless Chrome; menus, pickers, dialogs and toasts open.