Document the AdminAttribute and the AdminController naming convention

[MikeAlhayek]

To render the controller using the admin theme, you can either name your controller AdminController or decorate your controller using [Admin] attribute.

This is nowhere to be found in our documentation.

maybe we should add /reference/themes folder to the OrchardCore.Docs project. Then move the /guides/create-admin-theme/ folder to it.

Also, add a document for Themes and how they work by explaining IThemeService, IThemeManager and IThemeSelector