filestore docs: BlockList and Rebase logic (#2833)

doc/filestore/design/storage/tablet/block_list.md

@@ -0,0 +1,7 @@
+# Block List format and Rebase logic
+
+[MixedBlocks](data_index.md#mixedblocks) stores block metadata for each blob in the form of [BlockLists](https://github.com/ydb-platform/nbs/blob/main/cloud/filestore/libs/storage/tablet/model/block_list.h). BlockLists have complex binary format and are encoded by [this code](https://github.com/ydb-platform/nbs/blob/main/cloud/filestore/libs/storage/tablet/model/block_list_encode.cpp). CommitIds of the blocks described by BlockLists can be changed by the [rebase logic](https://github.com/ydb-platform/nbs/blob/main/cloud/filestore/libs/storage/tablet/rebase_logic.cpp) to improve metadata grouping and decrease BlockList size.
+
+Deletions in BlockList code simply mean block MaxCommitIds and are not directly related to DeletionMarkers.
+
+![mixed_blocks_svg](../../../excalidraw/mixed_blocks_rebase_logic.svg)

doc/filestore/design/storage/tablet/data_index.md

@@ -26,6 +26,7 @@ It is the main layer intended for long-term data storage.
 * Each blob may contain blocks belonging to multiple different inodes.
 * Each range is supposed to contain up to 4MiB of data (sometimes slightly more due to hash collisions).
 * Each write generates DeletionMarkers which are later applied to the MixedBlocks table via the [Cleanup operation](https://github.com/ydb-platform/nbs/blob/main/cloud/filestore/libs/storage/tablet/tablet_actor_cleanup.cpp).
+* Block metadata for each blob is stored in a [BlockList](block_list.md).
 
 ![mixed_blocks_svg](../../../excalidraw/mixed_blocks.svg)