Get Action Mailer guide ready for prime time

I noticed that the Action Mailer guide still has a WIP tag. I find the information provided in the guide very useful and also complete. As it is hidden in the navigation it is very easily overlooked. As I find the guide very useful I think we should no longer hold it back.

I did some minor edits:
* removed testing from the index as it is covered in the testing guide. The section still exists but merely to link to the testing guide.
* linked the mentioned RFC
* replaced usages of `user` with `@user` if `@user` is available
* linked to the Action Mailer section in the configuring guide

I also removed the WIP flag in `documents.yml`