Improve primary and secondary data explanation

In #366 we updated and rewrote the section on coupling data.
However, the difference between primary and secondary data still seems to be unclear.

A suggestion by @MakisH was to add a table to the final example to make things easier to understand.

| Category | Serial Coupling | Parallel Coupling |
| --- | --- | --- |
| Exchanged | `Displacements`, `Velocities`, `Forces` | `Displacements`, `Velocities`, `Forces` |
| Accelerated | `Displacements`, `Velocities` | `Displacements`, `Velocities`, `Forces` |
| Define coefficients/primary data | `Displacements` | `Displacements`, `Forces` |
| Not accelerated | `Forces` | - |

The question is if this helps to understand what is going on, or if this means that the section needs more work.