Document usage of config files

[tdurk93]

In CONTRIBUTING.md, we should add an introductory explanation of the various configuration files we use (.secureli.yaml, .secureli/repo-config.yaml, and .pre-commit-config.yaml). This should include the reasoning for splitting configuration across these files, and how they differ from each other.

[dlolaogundipe]

Core epic

[itoltz]

.secureli/.pre-commit-config.yaml is a config file used by the pre-commit library. I added info about it to the README in #560.

.secureli.yaml is already described in some detail in the README.

AFAIK there isn't any documentation on .secureli/repo-config.yaml.

I don't think it makes sense to add any of the config file documentation to CONTRIBUTING.md. That file is used for providing info to people who want to contribute to the seCureLI project; config file documentation is relevant to end users of seCureLI, and so should probably be in (or be linked from) the README.