[Docs] Add example for `headers`

Documentation for custom routes was released some days ago (https://github.com/vercel/next.js/pull/14887). We should also add one example for every documentation page to better show how it works.

The following three examples are required:

• rewrites For docs/api-reference/next.config.js/rewrites.md
• redirects For docs/api-reference/next.config.js/redirects.md
• headers For docs/api-reference/next.config.js/headers.md

Every example should contain very basic content and focus only on the feature it’s about, showing how it works for multiple use cases that are currently not covered by the documentation. After the example is complete, add a link to it at the beginning of the page in docs.

Feel free to open a PR and I’ll review it asap.

[styfle]

Looks like these are all complete now, closing 👍

[lfades]

@robintom If possible please make a PR per example, don’t merge all of them on a single one.