Add documentation or migrate to dartdoc specifications

[royarg02]

Current documentation, if any, in the app is of the following form:
https://github.com/RoyARG02/soil_moisture_app/blob/d6c1be02dfc4846ac22ba48c6870720529c66981/lib/pages/Credits.dart#L1-L7

It would be helpful to add more documentation or change existing documentation to dartdoc specifications.