add voxel regions layer chunk structure specification

doc/specs/region_voxels_chunk_spec.md

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+# Voxels Chunk (version 1)
+
+Voxel regions layer chunk structure.
+
+Values are separated for extRLE8 compression efficiency.
+
+File format BNF (RFC 5234):
+
+```bnf
+chunk    = (65536*byte)    block indices (most significant bytes)
+           (65536*byte)    block indices (least significant bytes)
+           (65536*byte)    block states (most significant bytes)
+           (65536*byte)    block states (least significant bytes)
+
+byte     = %x00-FF         8 bit unsigned integer
+```
+
+65536 is number of voxels per chunk (16\*256\*16)
+
+## Block state
+
+Block state is encoded in 16 bits:
+- 0-2 bits (3) - block rotation index
+- 3-5 bits (3) - segment block bits
+- 6-7 bits (2) - reserved
+- 8-15 bits (8) - user bits