[DX][WCM] 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 think this can't belong here, because this is *component* documentation (so there is no service container).

If we're going to talk about manually validating a password, it would need to be in the book or cookbook. My preference would be to move the entire `Encoding the User's Password` section from the book to a cookbook called `Encoding and checking the User's Password`. There, we would have the old `Encoding the User's Password` section + this new section about validating the user's password.

@WouterJ and everyone else, what do you think?