Add doc describing gui doc to allow topic linking

docs/modules/contributions/pages/gui/topic_contributions.adoc

@@ -1,6 +0,0 @@
-= Enter Document Title Here
-mitm001
-:description: GUI contributed libraries for the jmonkey engine.
-:keywords: gui, documentation, input, control, hud, contributions
-
-The official engine GUI library is xref:core:Nifty Gui.

docs/modules/contributions/pages/gui/topic_gui_contributions.adoc

@@ -0,0 +1,7 @@
+= GUI Contributions
+mitm001
+:description: GUI contributed libraries for the jmonkey engine.
+:keywords: gui, documentation, input, control, hud, contributions
+
+The official engine GUI library is xref:core:gui/nifty_gui.adoc[Nifty Gui]. This
+topic contains user contributed GUI libraries.