Add category for Publishing/Exports

Under Help, I think it would be useful to have a category for Publishing/Exports. It may even make sense to have Help be purely for the core Godot game development help and have Publishing/Exports be outside of the main Help category. That way it could be divided up into subcategories for each of the platforms (Android, iOS, etc.) as well as a subcategory for marketing questions that seem to come up.