Expose Collectives tags to unified search

[noantiq]

Is your feature request related to a problem? Please describe. When I use the unified search to search for a Collectives page, no page is found when typing in the name of a tag that I gave to multiple pages.

Describe the solution you'd like I want to be able to search for a tag across multiple collectives and get all pages that are tagged accordingly. That could be by selecting the tag, as it is currently implemented in Collectives or simply (more or less) adding the tag name to the content of a page, making it appear in the results of "Collectives - Page content".

Describe alternatives you've considered I can use the search inside a collective to search for a tag, but only for one collective at a time.

Additional context -

[max-nextcloud]

Pages are added to the search based on their content by the PageContentProvider which in turn uses the IndexedSearchService:

https://github.com/nextcloud/collectives/blob/51b7acbb98419b9a034820d3deeba0589ce9aaff/lib/Search/PageContentProvider.php#L78

So what's missing here is indexing the tags alongside the rest of the content. The content is currently indexed here:

https://github.com/nextcloud/collectives/blob/51b7acbb98419b9a034820d3deeba0589ce9aaff/lib/Search/FileSearch/FileIndexer.php#L89-L100

I don't know how the Collection could be extended. Maybe one could simply add to the content.