Nextcloud Collectives improvements [Hub 3]

Nextcloud Collectives introduces:
User mentions
Table of contents
Ability to link to headers between documents
Improved markdown compatibility and frontmatter support
Improved page management with drag and drop & custom sort orders
Improved print and export options
Support for inserting files in documents
Support for widgets for links, providing rich previews
Fulltext search using the Universal Search function in Nextcloud