More around design documents, add save design doc

design/ideas.md

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+Various random ideas that aren't (yet) part of the design
+
+- Character is 1.5 blocks tall
+- Can crawl to fit inside 1 block tall gaps
+
+
+- Multiple tile layers.
+    - Back layer
+    - Wall layer
+    - Ore/misc insert layer
+
+- No world hopping.
+  Characters are worlds are linked together, like in Minecraft.
+
+- No respawn after quit&save. Player character continues where it left off.