Moved "Impersonating a User" to a cookbook entry

Hi,

I've been looking to ways of shortening Security book entry, which is pretty impressive.
`Impersonating a User` seems to be a less common case, and in no way understanding it is necessary to understand the Security component.
Then I moved it to its own cookbook entry, added a link to it at the bottom of the Security book, updated indexes.

Its content was not modified, nothing new, nothing fixed. But I hope it'll make contributing to it easier :)