[DX] New service to simplify password encoding

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | [#11306](https://github.com/symfony/symfony/pull/11306)
| Applies to    | 2.6

Add documentation for symfony/symfony PR [#11306](https://github.com/symfony/symfony/pull/11306)

New service to simplify password encoding.
I would say, put it in the book chapter for now and create a new issue about splitting them. (I agree with splitting them)