Docs: Explain different export formats of next.config.js

[mrienstra]

What is the improvement or update you wish to see?

Info about various forms next.config.js can take in addition to the default (mjs, function, async function) should move from
/docs/app/api-reference/next-config-js/pageExtensions to /docs/app/api-reference/next-config-js.

Is there any context that might help us understand?

Prior to #50701, the info I think should be moved was located in docs/api-reference/next.config.js/introduction.md, but shown at https://nextjs.org/docs/app/api-reference/next-config-js/pageExtensions. See my previous issue (which predates #50701): #50192

As part of #50701, this content was moved to docs/02-app/02-api-reference/05-next-config-js/pageExtensions.mdx (permalink).

I think it would be more useful / intuitive if it was in docs/02-app/02-api-reference/05-next-config-js/index.mdx (permalink).

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Aside (I can open another issue for this if desired): documentation of defaultConfig is difficult to locate, not on either of these pages, and not found via search. Just a simple example would be nice, as is done for phase.

_DX-1709

[balazsorban44]

Thanks, I do agree that the https://nextjs.org/docs/app/api-reference/next-config-js page could explain a bit more about the supported formats.

In addition, each subpage/config property could mention its default value. I added this to our backlog