Deprecated ActionMailer .deliver and updated ActionMailer / ActiveJob docs/guide

- `SomeMailer.some_email(*args).deliver` is deprecated in favour of `.deliver_now`. (context #16549)
- Updated / Added rdoc for ActionMailer(Base, MessageDelivery)
- Updated guides, changelog, readme, release notes
We don't need to tell which class is. Even more because we are not documenting it.