Document SimpleVoidGen setup for Multiverse Core

Added instructions for setting up Multiverse Core with SimpleVoidGen.

Author: TinyBrickBoy

docs/simplevoidgen/Multiverse Core.md

@@ -0,0 +1,7 @@
+# SimpleVoidGen – Setup Multiverse Core
+
+## Create a new world with void generation
+
+```
+/mv create <world_name> <env> -g SimpleVoidGen -t FLAT
+```