Move functional concepts docs to Cargo docs

[tijlleenders]

Is your feature request related to a problem? Please describe.
Documentation should be in one place to make it more maintainable and accurate.
For a Cargo crate, this place is the Rust cargo docs.

Describe the solution you'd like
Move all the concepts and definitions on documentation/functional/Concepts.md to Rust structs and corresponding cargo docs.

Describe alternatives you've considered
Copy-Paste. This is more work and error-prone.

Additional context
Need came up when making #409