Search across multiple sites or indices

[olets]

A follow-up to #20:

Everything that moved from the VuePress docs site to the Ecosystem site no longer shows up in the VuePress docs site's search. That makes the content on Ecosystem hard to discover if you don't know to follow searches with going Reference > Ecosystem > click a link > repeat search.

It'd be cool if the VuePress docs site's search searched the VuePress docs site and the Ecosystem sites.

Non-DocSearch Algolia can do multi-index search (for example: multi-index InstantSearch https://www.algolia.com/doc/guides/building-search-ui/ui-and-ux-patterns/multi-index-search/js/). But I don't have much experience with DocSearch, and don't know if it can, or if it can build an index from multiple sites.