Check that the public interface of the crates is documented

[andreeaflorescu]

The proposal here is to add an integration test that builds the Firecracker binary using RUSTFLAGS="-D missing-docs" cargo build. It might be that there is another method to create a configuration per crate, but I didn't invested much time in this.

[andreeaflorescu]

The current solution for crates where we added documentation is to prefix the lib file with #![warn(missing_docs)]. One of the crates where we have this in place is vmm. Check the vmm/src/lib.rs for details. The missing docs directive will generate warnings when building Firecracker if it finds undocumented public code.

[andreeaflorescu]

Related to https://github.com/firecracker-microvm/firecracker/issues/609.

[alindima]

We are currently missing documentation for: devices, utils and vm-memory crates, as well as src/mmds/src/ns.rs. We should also add the following directive to each of them: !#[deny(missing_docs)]