docs: add flat storage struct docs (near#8450)

docs/architecture/flat_storage.md

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+Here we describe structures used for flat storage implementation.
+
+## FlatStorageState
+
+This is the main structure which owns information about ValueRefs for all keys from some fixed 
+shard for some set of blocks. It is shared by multiple threads, so it is guarded by RwLock:
+
+* Chain thread, because it sends queries like:
+    * "new block B was processed by chain" - supported by add_block
+    * "flat storage head can be moved forward to block B" - supported by update_flat_head
+* Thread that applies a chunk, because it sends read queries "what is the ValueRef for key for block B"
+* View client (not fully decided)
+
+Requires ChainAccessForFlatStorage on creation because it needs to know the tree of blocks after
+the flat storage head, to support get queries correctly.
+
+? - rename to FlatStorage
+
+## FlatStateFactory
+
+It holds all FlatStorageStates which NightshadeRuntime knows about and:
+
+* provides views for flat storage for some fixed block - supported by new_flat_state_for_shard
+* sets initial flat storage state for genesis block - set_flat_storage_state_for_genesis
+* adds/removes/gets flat storage if we started/stopped tracking a shard or need to create a view - add_flat_storage_state_for_shard, etc.
+
+? - rename to FlatStorageManager
+
+## FlatState
+
+Interface for getting ValueRefs from flat storage for some shard for some fixed block, supported
+by get_ref method.
+
+? - rename to FlatStorageSnapshot
+
+## FlatStorageCreator
+
+Creates flat storages for all tracked shards or initiates process of background flat storage creation if for some shard we have only Trie but not FlatStorage. Supports update_status which checks background job results and updates creation statuses and should create flat storage when all jobs are finished.
+
+## Other notes
+
+### Chain dependency
+
+If storage is fully empty, then we need to create flat storage from scratch. FlatStorageState is stored
+inside NightshadeRuntime, and it itself is stored inside Chain, so we need to create them in the same order
+and dependency hierarchy should be the same. But at the same time, we parse genesis file only during Chain
+creation. That’s why FlatStateFactory has set_flat_storage_state_for_genesis method which is called 
+during Chain creation.
+
+### Regular block processing vs. catchups
+
+For these two usecases we have two different flows: first one is handled in Chain.postprocess_block,
+the second one in Chain.block_catch_up_postprocess. Both, when results of applying chunk are ready,
+should call Chain.process_apply_chunk_result → RuntimeAdapter.get_flat_storage_state_for_shard → 
+FlatStorageState.add_block, and when results of applying ALL processed/postprocessed chunks are ready,
+should call RuntimeAdapter.get_flat_storage_state_for_shard → FlatStorageState.update_flat_head.
+
+(because applying some chunk may result in error and we may need to exit there without updating flat head - ?)