intro the var-dumper component
review effect
Reduce line length
doc for Dumpers
split intro/advanced sections
use VarCloner instead of Php/ExtCloner
move Dumper section above the Caster one
section for DebugBundle/Twig integration
§ about Data::getLimitedClone()
§ about DebugBundle config
minor adjustments
how to dump as string note
more accurate doc about limits
add examples
feature #4243 [WIP] var-dumper component (nicolas-grekas)

This PR was merged into the master branch.

Discussion
----------

[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

Commits
-------

041e858 add examples
23712fb more accurate doc about limits
c578fe7 how to dump as string note
edb0ff9 minor adjustments
6a3e170 § about DebugBundle config
cb0ee89 § about Data::getLimitedClone()
ee2d7e4 section for DebugBundle/Twig integration
b5b45bc move Dumper section above the Caster one
2fc3811 use VarCloner instead of Php/ExtCloner
350e0c7 split intro/advanced sections
036edcb doc for Dumpers
7dc6e5c Reduce line length
5e0b9bd review effect
26edf7a intro the var-dumper component