Create article explaining difference between federated and multi-index search

[guimachiavelli]

The article should:

• explain what is multi-search
• explain what is federated search
• describe use cases each one

As framed by @macraig and @Strift during a meeting:

• multi-search is helpful when users are likely to want to know where a result comes from. For example, if I search for shygirl on Spotify, I will probably appreciate seeing the artist's individual songs, albums, and collaborations displayed separately
• federated search is helpful when the provenance of a result is not important. For example, if I'm looking for Makoto's email in a CRM application, I don't care whether Meilisearch found it on her profile or on a chat log