Document the high-level design of argopy

We should provide a high-level design of argopy:
- in order to follow [FLOSS good practices](https://github.com/coreinfrastructure/best-practices-badge/blob/main/docs/other.md#documentation),
- to make it easier for new contributors to get started.

> The project MUST include documentation of the architecture (aka high-level design) of the software produced by the project. If the project does not produce software, select "not applicable" (N/A). (URL required).
> A software architecture explains a program's fundamental structures, i.e., the program's major components, the relationships among them, and the key properties of these components and relationships.