Google Docs Add-on

Google Docs Add-ons are customized integrations built with Apps Script that extend the editor's native capabilities with custom menus, sidebars, and automated workflows.

Google Docs Add-ons allow developers to customize the document editing experience by building tailored workflow improvements and third-party integrations directly inside the editor. Built on Google Apps Script, these tools leverage the Document service to programmatically read, format, and modify text, tables, and images. Developers can design custom sidebars and dialogs using standard HTML and CSS, and configure triggers to execute code automatically when a document opens. Once built, these integrations are published to the Google Workspace Marketplace, making it simple to deploy customized productivity tools to individual users or across entire enterprise domains.

https://developers.google.com/apps-script/add-ons/editors/docs