Allow searching across multiple categories

[JammyDodger]

This will be possible with the new filters.

Experimental topics list filter feature

We have recently introduced an innovative /filter route, enabling more sophisticated filtering of the topics list. Significantly, this marks the initial step towards a unified topic filtering language for both search and filtering. Additionally, the dedicated route for topic list filtering enhances the customizability of the sidebar, as users can now incorporate links as custom section links within the sidebar. How to turn it on Enable the experimental_topics_filter site settings in the admin i…

Still experimental though, but live on Meta if you want to give them a try.