docs: add introductory documentation for all crates

[nanderstabel]

Description of change

This change adds an introductory documentation for all the crates, basically setting the context for each library by referring their respective specification documents and how the different libraries relate to one another.

The previous code example was outdated so it is completely removed in this current change. Up to date code examples will be introduced again in a future update (after some some refactoring --> #56).

Links to any relevant issues

Mentioned in #43: 'individual crates should be easy to understand for first-time users'. This PR ads more context to the different crates. In a future change there will be diagrams and code examples added to the documentation as well.

How the change has been tested

n/a

Definition of Done checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes