Federated tutorial and explanation

[guimachiavelli]

Closes #2977, #2978

[meili-bot]

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[dureuill]

While searching across multi indexes is definitely a use case of federated search, I find that the guide and documentation makes it appear as if it was the only use case.

I find important to make it clear that federated search can also happen with multiple queries targeting the same index.

[guimachiavelli]

Thanks for the comments, @dureuill. I was only vaguely aware of the possibilities you outline, so that was helpful. I'll probably still focus the content on the use cases marketing/product want to promote, but I'll update the explanation article so users are at least aware of other possible uses.