docs(tip): add `package.json` tip to improve a prettier setup

[filiphsps]

Let's try that again, apparently GitHub weren't too thrilled about me renaming the branch of an active PR. ~~I'll fixup the build lengthen the description when I get back to a terminal later, doesn't hurt to get it under a few eyes a bit earlier though!~~

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Seen a lot of people get this wrong, so let's add a tip explaining the benefits of setting the prettier configuration in package.json rather than .prettierrc or prettier.json.

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