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
I believe it is better to remove the part where we enforce that users should not create it. If users want to create they will despite our recommendation or not :smile:. Also the `initialize` method is already with nodoc.