Allow to make selection automatically based on one (or a few) keyword

[kelson42]

That would make things very easy to build thematic selection. In addition we could add a parameter to limit the number of article to a maximum.

We could already provide such a feature by:

• Linking a Wikipedia to it's online version on http://library.kiwix.org
• wikifr code would then match https://library.kiwix.org/viewer#wikipedia_fr_all_maxi
• Then use the opensearch API of the fulltext index, see https://kiwix-tools.readthedocs.io/en/latest/kiwix-serve.html#search-searchdescription-xml to retrieve the article_id for the selection