[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.
Hmm, now that I think about it, I also think that this is an "edge case". But, one use might be if you want the user to type in their old password to change to a new one or something similar.

So let's keep this here. But I think we need to revisit these chapters later and maybe move some stuff around.