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/c11-progress-callback.md

@@ -56,4 +56,4 @@ auto res = cli.Get("/large-file",
 
 > **Note:** `ContentReceiver` and the progress callback can be used together. When you want to stream to a file and show progress at the same time, pass both.
 
-> For a concrete example of saving to a file, see [C01. Get the response body / save to a file](c01-get-response-body).
+> For a concrete example of saving to a file, see [C01. Get the response body / save to a file](../c01-get-response-body).