Update docs for NativeMenu + NativeMenuBar

[ewrogers]

I am new to Avalonia, but quite experienced with WPF and develop using Rider on macOS.

While developing my UI, I wanted to support the native menu bar at the top of the screen. Going through the documentation, I could not find anything about this. Looking in the Controls section I found NativeMenu but it is not clear how it works.

Then I found an older Github issue that mentions NativeMenuBar but that's not in the docs. Ultimately, I searched the entire repo for this and found an integration test example that uses it in XAML.

Using that I got it to work, but it shows Avalonia Application as in the macOS menu bar instead of my window title. Still not sure how/where to change that.

It would be really nice to update the docs with how to use NativeMenuBar and NativeMenu for cross-platform UIs.

[timunie]

@ewrogers transferred this into the Docs Repo. It's a good content request 👍

We will update the docs for 11.0 first, but it's noted at least.