Add ComboBox to docs

[londospark]

I've been playing around with the ComboBox in Avalonia and it seems to work the same way for my purposes as the ListBox does which is already documented, that being said: would it be worth documenting the ComboBox control in the same manner? Almost to the point of doing a copy/paste and rename job.

[JaggerJo]

That's because both are an SelectingItemsControl / ItemsControl underneath 🙂

Maybe we should have documentation on that.