Python: Workflow concurrency

We currently don't have a complete story for workflow concurrency.

Based on offline discussions, we should implement the following to support full workflow concurrency:
1. Support executor factories in workflow builder.
2. Make it clear that a workflow instance should not be run concurrently.
3. Make it clear that if concurrency is desired, use the workflow builder that only has executor factories (unless an executor is stateless or idempotent) to create fresh workflow instances.

Start by locating the Python workflow builder and executor-related entry points, then review the offline concurrency discussions referenced in the issue. Done means executor factories are supported, workflow instances are clearly documented as non-concurrent, and the factory-only builder guidance explains how to create fresh instances for concurrent runs.

Written by the indexing model from the issue text.