[ci skip] add guide for ActiveModel::SecurePassword

As asked in the #15494, this is the PR for the `ActiveModel::SecurePassword` guide.
I would rephrase this paragraph like this:

> `ActiveModel::SecurePassword` provides a way to securely store any password in an encrypted form. Including this module, a `has_secure_password` class method is provided which defines an accessor named `password` with certain validations on it.

What do you think ?