Define general UX/UI around searching a ZIM in readers

As time went by, all readers have developed their own UX/UI around the selection of a ZIM in the library.

While we recognize that it is important that all readers provide a native experience (searching on an Apple phone is obviously different than on a PWA on Windows), we are convinced that some general guidelines / recommendations would help to provide a similar experience and take whole benefits of all ZIMs metadata.

Some topics to consider:
- what is the entrypoint (a single search box, a huge UI with many filters available, ...)
- how do we guide users once they've a search result (e.g. avoid confusion between flavors, make ZIM date clearly visible, ...)
- how do we help users discover similar content (by using category for instance)
- how do we allow user to refine their search (through flavor, category, source, reserved tags, ...)
- how do we build "permalinks" in web apps (e.g. it is very useful to be able to have a link leading to all Wikipedia ZIMs in French to be used on zimit.kiwix.org when someone requests a ZIM of fr.wikipedia.org)

No files, tests, or entry points are named; start by reviewing how the existing readers handle ZIM selection and metadata. Produce agreed cross-reader UX/UI guidelines covering entry points, result guidance, discovery, refinement, and web-app permalinks.

Written by the indexing model from the issue text.