Write "transactions" subpage for Architecture Guide

Part of this ticket should be in figuring out where checkpoints and rollback to stable should be covered.  They could be subpages of transactions. Or they could be subpages of the top level architecture guide (perhaps with references from the transactions page).  Rollback to stable already has its own ticket: WT-6567. Wherever checkpoint ends up should probably incorporate WT-6667 and WT-5479.