7181: Document project_model::PackageData and project_model::TargetData r=arnaudgolfouse a=arnaudgolfouse
This PR adds some documentation for the `project_model` crate.
Some of the field descriptions were taken directly from their `cargo_metadata` counterpart :
- `PackageData` -> `cargo_metadata::Package`
- `TargetData` -> `cargo_metadata::Target`
Co-authored-by: Arnaud <arnaud.golfouse@free.fr>