Add Floating Islands and material-driven Age features (#45)

Keep repeated feature and material pages in authored writing, vary their resolved materials, and expose Link Panel modifier variants in creative and JEI. Remove Descriptive Book writing notes and refine Void biome selection.

src/main/resources/data/rivencraft/rivencraft/symbols/feature/floating_islands.json

@@ -0,0 +1,8 @@
+{
+  "schema_version": 2,
+  "name": "symbol.rivencraft.feature.floating_islands",
+  "description": "symbol.rivencraft.feature.floating_islands.description",
+  "category": "rivencraft:feature",
+  "semantics": { "role": "feature", "value": "rivencraft:floating_islands" },
+  "poem": "Terrain Elevation Island Sky"
+}