[ci skip] add guide for ActiveModel::SecurePassword

As asked in the #15494, this is the PR for the `ActiveModel::SecurePassword` guide.
Sorry I missed this one - Maybe "dependency" is a bit redundant. What do you think about:

> This maximum length of a password is 72 (required by `bcrypt-ruby` on which `ActiveModel::SecurePassword` depends)

? Maybe we can also use "a password" or `password` instead of "Password" for the list items ?