Document Signed HTTP exchanges

### MDN URL

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers

### What specific section or headline is this issue about?

_No response_

### What information was incorrect, unhelpful, or incomplete?

Follow up from: https://github.com/mdn/content/issues/1458

Need pages for the following:

- `Accept-Signature`
- `Signed-Headers`
- `Signature`

### What did you expect to see?

New pages

### Do you have any supporting links, references, or citations?

Also see Brian's resources:

> For signed exchanges, some info here:
>
> - https://github.com/WICG/webpackage?tab=readme-ov-file#specifications
> - Blogs:
> - https://web.dev/articles/signed-exchanges
> - https://developers.google.com/search/docs/appearance/signed-exchange
> - https://developers.cloudflare.com/speed/optimization/other/signed-exchanges/

### Do you have anything more you want to share?

_No response_