Create "why whisk" section in readme and docs

[AdamBarnhard]

I've seen a few OSS projects that have a "why X" section that is helpful to give an intro into the project. Im thinking we add a quick one to the readme and potentially a section to the docs as well.

[AdamBarnhard]

Related feedback from L.R.:

• It might be helpful to add a bit more detail around how whisk is different from similar projects (if applicable)
• It might be helpful to explain the use cases that whisk is and is not intended for. For example, whisk doesn’t seem like the right tool for managing the data pipelines / ML workflows to retrain models in production or make batch inferences
  • It seems like model retraining could be managed using src/features and src/models combined with a task orchestration platform like Airflow, but that doesn’t seem like the primary use case