Add simple tutorials

- getting started from scratch (adding dependency, creating a config reader with env vars, reading from it)
- migration tutorials:
- from pure env vars
- from an existing YAML file

Start by reading the existing documentation for the Swift configuration API and its dependency setup. Cover getting started from scratch, including environment variables, then add migration tutorials for pure environment variables and an existing YAML file. Done means each path is documented clearly enough to follow from setup through reading configuration.

Written by the indexing model from the issue text.