[#3491] Moving the new named algorithms into their own cookbook entry

Hi guys!

This makes some changes to #3491:

* moves the entry into a cookbook entry (to try to keep the security as short as possible)
* tweaks inside the entry

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes, PR symfony/symfony#10005 - but no, this is just a modification of #3491 
| Applies to    | 2.5+
| Fixed tickets | -

Thanks!