docs: port Flask tutorial from Juju.is (canonical#1976)

The current Flask tutorial:
https://juju.is/docs/sdk/write-your-first-kubernetes-charm-for-a-flask-app

This PR will move the tutorial off of Discourse and Charm SDK and onto
the Charmcraft RTD.

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Co-authored-by: Michael DuBelko <[email protected]>
Co-authored-by: Alex Lowe <[email protected]>