link directory to ActionMailer Previews documentation
[ci skip] Concludes changelog links in the bottom.
This way when we do next release notes, this would be easier to copy and change.
Merge pull request #16068 from JuanitoFatas/4-2-changelogs

[ci skip] Concludes changelog links in the bottom.
Merge pull request #16044 from jjb/patch-1

[ci skip] link directly to ActionMailer Previews documentation
active_record: Type cast booleans and durations for string columns.