Introduce dummy Shapes in the assembly docs

Splitting this off from #850 as it's own issue.

- [ ] Move that bit from the end of Assembly example -> Constraints definition:
> Should you need to do something unusual that is not possible with the string based selectors (e.g. use cadquery.selectors.BoxSelector or a user-defined selector class), it is possible to pass cadquery.Shape objects to the cadquery.Assembly.constrain() method directly.

to this new subsection.
- [ ] Show how to make dummy Shapes
- [ ] List some of the more useful methods (mainly `Vertex.makeVertex`, `Face.makePlane`) for creating dummy Shapes
- [ ] Note that similar results can be achieved with `Workplane.tag` and `Workplane.end` to leave shapes in the assembly object that are not rendered in the final assembly.