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/s07-multipart-reader.md

@@ -68,4 +68,4 @@ Only a small chunk sits in memory at any moment, so gigabyte-scale files are no
 
 > **Warning:** When you use `HandlerWithContentReader`, `req.body` stays **empty**. Handle the body yourself inside the callbacks.
 
-> For the client side of multipart uploads, see [C07. Upload a file as multipart form data](c07-multipart-upload).
+> For the client side of multipart uploads, see [C07. Upload a file as multipart form data](../c07-multipart-upload).