[WIP] var-dumper component

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.6
| Fixed tickets | none

- [x] general introduction
- [x] cloner doc
- [x] caster doc
- [x] dumper doc
- [x] config doc
- [x] example output with explanations

it is better to follow the coding standards of the symfony-docs repo, to be consistent with the whole content