Document where is MediaPlayerElement available

Problem to solve

Document on which platforms/framework is the MediaPlayerElement available in a table. For now, it is available on:

• Uno (UWP, Android, iOS),
• Xamarin.Forms (Android, iOS, macOS, WPF, GTK)

It is currently NOT available on standalone platforms:

• Xamarin.Android,
• Xamarin.iOS,
• Xamarin.tvOS,
• UWP,
• WPF,
• WinForms,
• GTK,
• macOS.

Intended users

LibVLCSharp consumers.

Proposal

Make a table.