docs: add chunk file format

Documentation/format/chunks.md

+# Chunks Disk Format
+
+The following describes the format of a single chunks file, which is created in the `chunks/` directory of a block.
+
+```
+ ┌────────────────────────────┬──────────────────┐
+ │ magic(0x85BD40DD) <4 byte> | version <1 byte> │
+ ├────────────────────────────┴──────────────────┤
+ │                     Body ...                  │
+ └───────────────────────────────────────────────┘
+```
+
+Available versions:
+* v1 (`1`)
+
+## Body (v1)
+
+The body contains a sequence of chunks, each of which has the following format.
+
+```
+┌─────────────────────────────────────────────────────────┐
+│ ┌──────────────┬───────────────────┬────────────┐       │
+│ │ len <varint> | encoding <1 byte> │    data    │  ...  │
+│ └──────────────┴───────────────────┴────────────┘       │
+└─────────────────────────────────────────────────────────┘
+```
+
+The length marks the length of the encoding byte and data combined.
+The CRC checksum is calculated over the encoding byte and data.
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