Add description for Disk Join Step. (#2882)

docs/DiskJoinStep.md

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+`DiskJoinStep(DJS).`
+
+`DiskJoinStep` is designed to handle large joins, which are exceeded the memory limit.
+CS uses "hash join" to implement a join.
+
+`HashJoin` - (small side table, large side table).
+Initialize hash table with small side, join large side row by row on a hash table, append result into result data list (FIFO).
+
+`Disk based hash join` - (small side table, large side table).
+Based on small side data create a "partition tree".
+Based on large side data use existing "partition tree" (without spliting) to store large side data.
+`Partition tree` - is a tree where each node holds a pointers to other nodes, except leaf nodes, they hold pointers to the file with data.
+Each leaf node holds data which gives the same hash, therefore we can join small side on large side only from the same leaf nodes.
+Partition tree can be splitted when the leaf node exceeded the memory limit for the small side.
+`Split leaf node` - make leaf node a regular node, redistribute the data in this node into new created nodes based on hash.
+
+
+```mermaid
+stateDiagram-v2
+[*] --> mainRunner
+state mainRunner {
+  [*] --> smallReader
+  smallReader --> largeReader: Read small data and create `partition tree`.
+  largeReader -->  collectSmallSidePartitions: Read large side data and create a large side `partition tree` based on small side `partition tree`.
+  collectSmallSidePartitions --> Loader: Collect leaf nodes from partition tree.
+  Loader --> Builder: Process leaf nodes. Read RGData from the file and send to the `Builder`.
+  Builder --> Joiner: Initialize a hash table with RGData.
+  Joiner --> largeReader: Read RGData from `partition tree` for large side and join.
+}
+```