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/c03-default-headers.md

@@ -52,4 +52,4 @@ auto res = cli.Get("/users", headers);
 
 Per-request headers are **added** on top of the defaults. Both are sent to the server.
 
-> For details on Bearer token auth, see [C06. Call an API with a Bearer token](c06-bearer-token).
+> For details on Bearer token auth, see [C06. Call an API with a Bearer token](../c06-bearer-token).