[DO NOT MERGE] 1.4.0

This is a WIP PR for 1.4 release notes.  I'll update it for each end of sprint and rebase it for the latest changes in the 1.3 manual.  At some point before RCs we can merge it and update it solely.

Please do review the changes particularly to `_includes/manuals/1.4/1.2_release_notes.md`.