chore(docs): add code sample to landing page

docs/guide/what-is-ys.md

@@ -11,6 +11,37 @@ Ys is a modern, minimal web application framework for the C programming language
 - [Zero-config CORS](../documentation/cors.md)
 - [Cookies](../documentation/cookies.md)
 
+```c
+#include "libys.h"
+// ...
+
+ys_response *handler(ys_request *req, ys_response *res) {
+  ys_set_header(res, "Content-Type", "text/plain");
+
+  ys_cookie *c = ys_cookie_init(COOKIE_ID, sid);
+  ys_cookie_set_expires(c, ys_n_minutes_from_now(10));
+  ys_set_cookie(res, c);
+
+  ys_set_body(res, "Hello World!");
+  ys_set_status(res, YS_STATUS_OK);
+
+  return res;
+}
+
+int main() {
+  ys_router_attr *attr = ys_router_attr_init();
+  ys_use_cors(attr, ys_cors_allow_all());
+  ys_router *router = ys_router_init(attr);
+
+  ys_router_register(router, "/", handler, YS_METHOD_GET);
+
+  ys_server *server = ys_server_init(router, PORT);
+  ys_server_start(server);
+
+  return EXIT_SUCCESS;
+}
+```
+
 ## Why?
 
 C programs tend to be verbose. You seldom see C servers in the wild because the development overhead is absolutely massive and mired in complexity; before you can think about building an API, you'll need to sift through system calls, parse raw requests, juggle socket descriptors, and become intimately familiar with the HTTP specification.