Add specific filter for search results in Algolia search

[chrisgitiota]

Is your feature request related to a problem? Please describe.
When using the wiki search I'm often only interested in results that relate to a specific topic like Hornet, WASP, iota-sdk and so on. Reviewing the search results I have to scroll through a lot of entries that relate to topics that I'm not interested in. This could be avoided if the search could be narrowed to specific topics.

Describe the solution you'd like
A possible solution could be to have a topic filter dropdown list on the "See all ### results" page. The result list then would be filtered in the browser. As the breadcrump of each search result is already shown for each result list entry, this should work but browser side filtering is OFC a little bit dangerous when it comes to very big result lists and page navigation (navigating backward and forward in the browser history).

Another option would be to integrate a topic filter section into the search overlay formular that is used to start the search. As this is already an overlay, a dropdown list would not be suitable here. May be we could add a "Filter" button and if the button is pressed, the overlay is enlarged and a topic filter section becomes visible.

[Dr-Electron]

@jlvandenhout as discussed today we could also think about prioritization of results. Not sure if we want to track that in an separate issue 🤔