Provide Dark theme switcher guidance for PF consumers

[andrew-ronaldson]

Providing a standard approach for Dark theme swtiching.

With dark theme now available in Patternfly 5 we should provide guidance on how it can be incorporated into a products UI.
Currently, the PF site uses the switch to component in the masthead, however, products may require a smaller solution that doesn't take as much screen space.

[andrew-ronaldson]

Closing this issue and created followup in Core with #3733