Fix broken relative links in cookbook docs (Fix #2490)

Cookbook body links referenced sibling pages with a bare slug
(e.g. `c14-keep-alive`). Under the pretty-URL layout each page lives
in its own directory, so these resolve against the page's own
directory and 404. Prefix them with `../` to match the convention
already used in the tour and llm-app sections.

Verified clean with `docs-gen check`.

docs-src/pages/en/cookbook/s05-stream-response.md

@@ -60,4 +60,4 @@ Call `sink.done()` to signal the end.
 
 > **Note:** The provider lambda is called multiple times. Watch out for the lifetime of captured variables — wrap them in a `std::shared_ptr` if needed.
 
-> To serve the file as a download, see [S06. Return a file download response](s06-download-response).
+> To serve the file as a download, see [S06. Return a file download response](../s06-download-response).