Allow inter-ZIM browsing

[Popolechien]

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.

[kelson42]

Never achieved to find a good approach to that kind of problem (for exemple to make working interwiki links in Wikipedia).