Similar to what we did on cywiki (T162244: [Story] Search index 10,000 article placeholders on cywiki for testing and evaluation purposes), we want to enable indexing for ArticlePlaceholders on nnwiki.

We need to decide how many placeholders we initially want to index (or do we just want to index all of them?).

Assigning to Lydia, as she will check if there are any objections from the community.