Docmost v0.21.0 adds zip, Notion, and Confluence imports

Docmost v0.21.0 adds major import options including zip files, Notion, and Confluence (Enterprise Edition) to facilitate content migration. The release also introduces generic iframe embed support, user preferences for read and edit modes, and workflow enhancements such as creating new pages from @ mentions. Additional improvements include updated table menu options, persistent Excalidraw libraries, Ukrainian translation, and bug fixes.