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/w01-websocket-echo.md

@@ -71,7 +71,7 @@ ws.send("Hello");                        // text frame
 ws.send(binary_data, binary_data_size);  // binary frame
 ```
 
-The `std::string` overload sends as **text**; the `const char*` + size overload sends as **binary**. A bit subtle, but once you know it, it's intuitive. See [W04. Send and receive binary frames](w04-websocket-binary) for details.
+The `std::string` overload sends as **text**; the `const char*` + size overload sends as **binary**. A bit subtle, but once you know it, it's intuitive. See [W04. Send and receive binary frames](../w04-websocket-binary) for details.
 
 ## Thread pool implications
 
@@ -83,6 +83,6 @@ svr.new_task_queue = [] {
 };
 ```
 
-See [S21. Configure the thread pool](s21-thread-pool).
+See [S21. Configure the thread pool](../s21-thread-pool).
 
 > **Note:** To run WebSocket over HTTPS, use `httplib::SSLServer` instead of `httplib::Server` — the same `WebSocket()` handler just works. On the client side, use a `wss://` URL.