Add docs about referencing subassemlies.

Using `partX/subpartY@...` is not mentioned anywhere in the docs:
```python
assy2 = (
cq.Assembly()
.add(subassembly, name="partX", color=cq.Color("orange"))
.add(part2, name="part2", color=cq.Color("red"))

#NB: partX/subpartX1
.constrain("partX/subpartX1@faces@>X", "part2?right", "Plane")
)
assy2.solve()
```