docs: new tutorial on converting a frontend data source to a backend data source

[sympatheticmoose]

Required sections:

• [ ] Query Data: should discuss moving from query in the frontend to QueryData in the backend and the creation of dataframes
• [ ] Health Checks: should discuss moving from testDatasource to CheckHealth
• [ ] Authentication
• [ ] Call Resource: should provide a generic introduction to CallResource and reference the proxy

Draft content (public): https://docs.google.com/document/d/1CcYzOJCrAvryzBDxo9kL5NFExZmCYtB3P7e1ihs8Pdo/edit