Improve & Document Code Structure

[Indy2222]

TODO

• Extend / update CONTRIBUTING.md to contain the following structure.
• Adapt the code based on the guidelines.
• Update & extend crate level documentation.

Structure

• Game functionality is split into many small crates with hundreds to thousands SLOC.
• Keep public API of each crate at minimum.
• Whenever feasible, the public API consists solely of events and system labels.
  • Documentation of each event contains:
    • stage in which the event originates / is consumed
    • system label (possibly labeling multiple systems) of the systems which send/consume the event
• Crate public API may expose components, however components may be inserted, removed or mutated solely from their respective crates.
• event struct names end with Event
• label enum names end with Label