www/rubygem-rails81: Add rubygem-rails81 8.1.0

Rails is a web-application framework that includes everything needed to create
database-backed web applications according to the Model-View-Controller (MVC)
pattern.

Understanding the MVC pattern is key to understanding Rails. MVC divides your
application into three layers: Model, View, and Controller, each with a specific
responsibility.

Active Record, Active Model, Action Pack, and Action View can each be used
independently outside Rails.

In addition to that, Rails also comes with:
- Action Mailer, a library to generate and send emails
- Action Mailbox, a library to receive emails within a Rails application
- Active Job, a framework for declaring jobs and making them run on a variety of
  queuing backends
- Action Cable, a framework to integrate WebSockets with a Rails application
- Active Storage, a library to attach cloud and local files to Rails
  applications
- Action Text, a library to handle rich text content
- Active Support, a collection of utility classes and standard library
  extensions that are useful for Rails, and may also be used independently
  outside Rails