Google Workplace Add-ons are now available for Google Docs, Sheets, and Slides. Google Workspace Add-ons are already available for Calendar, Gmail, and Google Drive.
There have been Editor Add-ons for a number of years, with Google Workspace Add-ons developers can tap into a new integrated user experience. One of the main differences with Workspace Add-ons is the use of the Card service rather than custom HTML (HTMLService). The benefit of switching to a Card service is cross platform support including integration into Google’s Workspace mobile apps.
Follow the link to the source post for links to all the relevant developer documentation.