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docs: add storage/addon-block documentation draft

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# Add-on Block
Fixed storage which stores an immutable binary-search tree of items.
Each item consists of a pair of pointers to binary data of arbitrary length.
These two pointers are interpreted as a `key` and a `value`.
Since items contain only pointers, multiple instances of data are stored just once.
Block contains 3 logical parts:
a) header (256 bytes)
b) binary-search tree (variable size)
c) item data (variable size)
## Header
| offset | length | name | description |
|-------:|---------:|--------------|--------------|
| 0x0000 | 4 | magic | magic `TRAB` |
| 0x0004 | 64 | sig | signature of the whole blob (except magic and sig) |
| 0x0044 | 4 | tree_count | number of elements in the tree |
| 0x0048 | 4 | items_size | length of the item section |
| 0x004A | 182 | reserved | - |
## Binary-Search Tree
Each node of the tree has the following structure:
| offset | length | name | description |
|-------:|---------:|--------------|--------------|
| ... | 2 | key_offset | key offset (divided by 4) |
| ... | 2 | value_offset | value offset (divided by 4) |
# Item Data
Each item has the following structure:
| offset | length | name | description |
|-------:|---------:|--------------|--------------|
| ... | 2 | item_len | item (key or value) length |
| ... | 2 | item_flags | item (key or value) flags |
| ... | item_len | item_data | item (key or value) data (padded to multiple of 4) |