Write initial Which Consensus? topic

Create a single topic that consolidates and summarizes consensus information (requirements, settings, tips, etc.) for devmode, PoET, PBFT, and Raft. Ideally, this topic should explain the pros/cons and best-practice tips for when to use each type of consensus.

This high-level topic may require several rounds of reviews and rewrites to fine-tune the information. This story covers the initial version and first round of reviews.