Add Credo, CI and some documentation

[odarriba]

This change adds several improvements related to code quality and documentation:

• Credo: it is now on the project, fully configured and passing all checks.
• Github Actions: when this change is merged, all commits will generate a CI execution that will check warnings, formatter, credo and tests.
• Documentation: I have added some documentation. Not to the extend I'd like but enough at the stage we are right now.

As an extra point, I have also made two more changes:

• Delete an unused module from the beginning of the project (ErrorTracker.Repo)
• Renamed the models folder to schemas

[crbelaus]

Ship it!