Move the PBKDF2 code to hashing section

[ghost]

When I read the header of encryption I think of something reversible, say using AES or the like, but the example specifically stated it's a hash. To me it would make more sense to move this code under the hashing section. Would this be something reasonable to adjust?

[AndyGauge]

It looks like we haven't done any encryption in the cookbook. I think that removing Encryption would be prudent.

[AndyGauge]

opened #562 to brainstorm encryption ideas.