[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.
Hey!

Move this into the `book/security.rst` page and remove it from the component docs completely :).

Then *later* (after this PR) we'll do some more work to move some things out of the book and into the cookbook. But you don't need to worry about that.

Thanks!