[ci skip] add guide for ActiveModel::SecurePassword

As asked in the #15494, this is the PR for the `ActiveModel::SecurePassword` guide.
Passwords are not *encrypted* (which is two way), they are *hashed/digested* (which is one way). I'll leave it to you to reword this :) While you're there "an encrypted form" is somewhat confusion because of HTML `<form>`s in this context, consider using "fashion", etc.