doc: improve a word client side rendering

[sumiren]

The term client-side rendering is ambiguous and misleading.
Some people would assume the architecture where a blank page is returned from the server and HTML generation is totally done on the frontend.

So I changed the term to client-side data fetching, which is widely used throughout the rest of the document.

Bug

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Documentation / Examples

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[ijjk]

This tweak does seem more accurate as we are strictly talking about data-fetching here and not rendering, thanks!