Allow inter-ZIM browsing

Some websites are cross-referencing each other (typically a link to Wikipedia, as in https://github.com/openzim/freecodecamp/pull/71). It would be a good QoL improvement for the reader(s) to be able to find these websites if they are also present locally, so that users could use (some) external links.

Whether these would point to the exact article rather than the other zim's homepage is a secondary, non-essential element in the early stage.

The issue names no files, tests, or entry points. Start by locating the reader's external-link handling and local ZIM discovery flow; done means users can open eligible external links when the referenced website is available locally, with exact-article targeting treated as optional.

Written by the indexing model from the issue text.