Lint for missing docs in all items #22

We want to uphold high quality, easy maintenance and enable contributors to become productive as fast as possible in trafix. To achieve this, we already have clippy::pedantic lints enabled to aid in good practices.

We also want to ensure that we provide good documentation. Public documentation is extremely important for users of the crate, and hence we should add warn(missing_docs). However, to aid in onboarding of contributors, ease of maintenance and general development experience, we should also add warnings for clippy::missing_docs_in_private_items lint.