[SFT] Add description for specifying SFT impl during snapshot recovery

HBASE-26286 added an option to clone_snapshot command that allows for specifying the SFT implementation during the snapshot recovery. This really useful when recovering snapshots imported from clusters not using the same SFT impl as the one where we are cloning it. Without this, the cloned/restore table will get created with the the SFT impl of its original cluster, requiring extra conversion steps using the MIGRATION tracker.

This also fix formatting for the "Bulk Data Generator Tool", which is currently displayed as a sub-topic of the SFT chapter. It should have it's own chapter.