Deprecate itemGroups field

Since 1.19.3, this was only populated when the player opened the
creative menu, and so was useless in survival or multi-player
worlds.

Rather than removing the field entirely (🦑 backwards compatibility), we
replace it with the empty list. We also remove it from the docs, and add
a note explaining what the field used to do.

Closes #1285, albeit in the least satisfactory way possible.

projects/fabric/src/main/java/dan200/computercraft/shared/platform/PlatformHelperImpl.java

@@ -256,12 +256,6 @@ public class PlatformHelperImpl implements PlatformHelper {
         return fuel == null ? 0 : fuel;
     }
 
-    @Nullable
-    @Override
-    public ResourceLocation getCreativeTabId(CreativeModeTab tab) {
-        return tab.getId();
-    }
-
     @Override
     public ItemStack getCraftingRemainingItem(ItemStack stack) {
         return stack.getRecipeRemainder();