[WIP] Release notes fixes

A series of fixes for the 4.1 release notes

Todo: 
* [x] Add Action Mailer previews to the release notes
* [x] Add secrets.yml to the release notes
* [x] Add concerning

I can squash and add [ci skips] after this has been reviewed, there are a few things I'm not too sure about (variants -> Action Pack request variant) so keeping them isolated makes it easier to revert those if needed.