More around design documents, add save design doc

design/save.md

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+Game has different worlds you can generate.
+Each world has its own folder.
+Each folder has:
+    - Player file (player.dat)
+    - Subfolder for regions (regions/)
+
+Each region subfolder has:
+r.x.y.dat files for each chunk, where x and y are the region coordinates.
+
+# Regions
+A region stores multiple chunks in a single file for more optimized storage.
+Current plan is to store 8 chunks in a region, in other words, 1024*1024 blocks.
+Around one million blocks, compressed with zstd compression.