docs: Add link to pageExtensions config in page-without-valid-component.md (#34285)
docs: Add link to pageExtensions config in page-without-valid-component.md (#34285)

The [Page Without Valid React Component](https://nextjs.org/docs/messages/page-without-valid-component) explains some ways to fix the problem of Next finding a non-React Component. However, if someone is trying to use `.mdx` as an extension, or wants to include tests, stories or other non-page files in their `pages` folder there's nothing for them.

The [Custom Page Extensions](https://nextjs.org/docs/api-reference/next.config.js/custom-page-extensions) has all the info to help them out, but adding it to this error page should make it more discoverable.



## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [x] Make sure the linting passes by running `yarn lint`

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