[WCM] var-dumper component

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.6
| Fixed tickets | none

- [x] general introduction
- [x] cloner doc
- [ ] caster doc
- [ ] dumper doc
- [ ] example output
- [ ] enhance the RST (with others, I'm far from a specialist)
IMO, the advanced usage should go in a separate file.

And the basic usage should give more details (a screenshot of the output for instance)