docs: add example for merging vite config defined as a function

[vinassefranche]

Description

As discussed in this comment, it's not trivial to understand how to merge a vite config defined as a function. Here's what I added concerning it:

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• It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
• Ideally, include a test that fails without this PR but passes with it.
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Tests

• Run the tests with pnpm test:ci.

Documentation

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Changesets

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