doc(#56): add a new slot content on the youtube slide in documentation

Author: peterpeterparker

docs/docs/slides/app-slide-youtube/app-slide-youtube.md

@@ -37,7 +37,7 @@ The "Youtube" slide's Web Component could be integrated using the tag `<deckgo-s
 
 ### Slots
 
-The slot `title` is optional.
+The slot `title` and `content` are optional. The slot `content` is displayed before respectively above the video.
 
 ### Notes