Add Cookbook other topics (draft)

docs-src/pages/en/cookbook/w02-websocket-ping.md

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+---
+title: "W02. Set a WebSocket Heartbeat"
+order: 52
+status: "draft"
+---
+
+WebSocket connections stay open for a long time, and proxies or load balancers will sometimes drop them for being "idle." To prevent that, you periodically send Ping frames to keep the connection alive. cpp-httplib can do this for you automatically.
+
+## Server side
+
+```cpp
+svr.set_websocket_ping_interval(30); // ping every 30 seconds
+
+svr.WebSocket("/chat", [](const auto &req, auto &ws) {
+  // ...
+});
+```
+
+Just pass the interval in seconds. Every WebSocket connection this server accepts will be pinged on that interval.
+
+There's a `std::chrono` overload too.
+
+```cpp
+using namespace std::chrono_literals;
+svr.set_websocket_ping_interval(30s);
+```
+
+## Client side
+
+The client has the same API.
+
+```cpp
+httplib::ws::WebSocketClient cli("ws://localhost:8080/chat");
+cli.set_websocket_ping_interval(30);
+cli.connect();
+```
+
+Call it before `connect()`.
+
+## The default
+
+The default interval is set by the build-time macro `CPPHTTPLIB_WEBSOCKET_PING_INTERVAL_SECOND`. Usually you won't need to change it, but adjust downward if you're dealing with an aggressive proxy.
+
+## What about Pong?
+
+The WebSocket protocol requires that Ping frames are answered with Pong frames. cpp-httplib responds to Pings automatically — you don't need to think about it in application code.
+
+## Picking an interval
+
+| Environment | Suggested |
+| --- | --- |
+| Normal internet | 30–60s |
+| Strict proxies (e.g. AWS ALB) | 15–30s |
+| Mobile networks | 60s+ (too short drains battery) |
+
+Too short wastes bandwidth; too long and connections get dropped. As a rule of thumb, target about **half the idle timeout** of whatever's between you and the client.
+
+> **Warning:** A very short ping interval spawns background work per connection and increases CPU usage. For servers with many connections, keep the interval modest.
+
+> For handling a closed connection, see W03. Handle Connection Close.