Updated Key Review deck guidelines for GCs

The existing Group Conversation presentation guidelines for functions with key reviews is as follows:

Functions with Key Reviews should not create a new deck for a Group Conversation but should use their Key Review deck for the Group Conversation.

However, this is often overlooked and functions choose to create a new slide deck instead. This seems to be an accepted approach, therefore I propose we update the guidance:

Functions with Key Reviews can use their Key Review deck for the Group Conversation. Optionally, you may choose to create new slides tailored for the wider Group Conversation audience.

This will (hopefully) remove existing ambiguity.