Mistral documentation: architecture

Implements blueprint: mistral-docs-general

Change-Id: I4c398a8677b8c0c6004755db989746f1e918ceea

doc/source/architecture.rst

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 Architecture
 ============
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+.. image:: img/mistral_architecture.png
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+* Engine - handles control and data flow of workflow executions. Computes which tasks are ready and places them in a queue. Passes the data from task to task, deals with condition transitions, etc.
+* Task Executors - execute task Actions. Pick up tasks from the queue, run actions, and send results back to the engine.
+* API server - exposes REST API to operate and monitor workflow executions.
+* Scheduler - stores and executes delayed calls. It is the important Mistral component since it interacts with engine and executors. It also triggers workflows on events (e.g., periodic cron event)
+* Persistence - stores workflow definitions, current execution states, and past execution results.