Support `/multi-search` (`.multiSearch(...)`) to allow cross-index search

[tats-u]

https://www.meilisearch.com/docs/reference/api/multi_search

The /multisearch (searchClient().multiSearch(...)) endpoint allows for displaying a single search result by mixing results from multiple search queries with prioritized weighting.

When a single site provides multiple types of documents with different indexes, it is reasonable for search to mix:

• Search results for the current document (high priority, main)
• Search results for other documents (low priority, supplementary)

while considering their priorities. However, the current /indexes/<index>/search (searchClient().index(index).search(...)) endpoint does not support searching across multiple indexes in this manner.

Question: how do we design the props of DocSearch(Modal) to switch from .index(...).search(...) to .multiSearch(..)?

[amrbashir]

We could either add DocSearchMultiSearch which is explicit or we could change indexUid in DocSearchProps to be string | string[] and in the case of it being an array we use .multiSearch(..)

I am leaning towards the latter.

[tats-u]

DocSearchMultiSearch

We want to reuse the logic in DocSearchModal except for a part of search() as much as possible. We sometimes want to adjust weight in federationOptions in each index in the "queries". q there can be a function (<search box string>: string) => string (value actually passed to q in API). If such a function is not passed, q should be the same as the search box string like the current single index search.

The argument of multiSearch is highly customizable, so it is likely to be difficult to come up with the best design.