Add consensus engine info to Sysadmin + App Dev Guides

Provide admin-specific information about the consensus interface for the System Administrator's Guide, with links to the dynamic consensus overview in the Introduction (and other consensus-related information as appropriate).  This info should summarize how to choose and configure a consensus engine.

For the App Dev Guide, add links to this information.

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PREVIOUS CONTENT

Provide appdev-specific information about the consensus interface for the Application Developer's Guide, with links to the consensus interface topic in the Architecture Guide.  For the App Dev Guide, this info should summarize how to choose and configure a consensus engine.

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ORIGINAL CONTENT

Describe new consensus interface (API, consensus engines-, SDKs-) based on the RFC, presentations, and other info sources.

This information is needed for the PBFT documentation: must briefly define/explain the Sawtooth consensus interface in once place, so that the consensus engine steps in the procedures can link to this information.

Consider using the current "Dynamic Consensus Algorithm" section in the Sawtooth introduction, and replacing the intro's section with a shorter summary.

Also document consensus fallback mechanism for existing chains on networks upgraded from 1.0. Implementing PR: https://github.com/hyperledger/sawtooth-core/pull/2056