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/ja/cookbook/c05-basic-auth.md

@@ -43,4 +43,4 @@ auto res = cli.Get("/private", headers);
 cli.set_digest_auth("alice", "s3cret");
 ```
 
-> BearerトークンでAPIを呼びたい場合は[C06. BearerトークンでAPIを呼ぶ](c06-bearer-token)を参照してください。
+> BearerトークンでAPIを呼びたい場合は[C06. BearerトークンでAPIを呼ぶ](../c06-bearer-token)を参照してください。