Add Cookbook S01-S22 (draft)

docs-src/pages/en/cookbook/s01-handlers.md

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+---
+title: "S01. Register GET / POST / PUT / DELETE Handlers"
+order: 20
+status: "draft"
+---
+
+With `httplib::Server`, you register a handler per HTTP method. Just pass a pattern and a lambda to `Get()`, `Post()`, `Put()`, or `Delete()`.
+
+## Basic usage
+
+```cpp
+#include <httplib.h>
+
+int main() {
+  httplib::Server svr;
+
+  svr.Get("/hello", [](const httplib::Request &req, httplib::Response &res) {
+    res.set_content("Hello, World!", "text/plain");
+  });
+
+  svr.Post("/api/items", [](const httplib::Request &req, httplib::Response &res) {
+    // req.body holds the request body
+    res.status = 201;
+    res.set_content("Created", "text/plain");
+  });
+
+  svr.Put("/api/items/1", [](const httplib::Request &req, httplib::Response &res) {
+    res.set_content("Updated", "text/plain");
+  });
+
+  svr.Delete("/api/items/1", [](const httplib::Request &req, httplib::Response &res) {
+    res.status = 204;
+  });
+
+  svr.listen("0.0.0.0", 8080);
+}
+```
+
+Handlers take `(const Request&, Response&)`. Use `res.set_content()` to set the body and Content-Type, and `res.status` for the status code. `listen()` starts the server and blocks.
+
+## Read query parameters
+
+```cpp
+svr.Get("/search", [](const httplib::Request &req, httplib::Response &res) {
+  auto q = req.get_param_value("q");
+  auto limit = req.get_param_value("limit");
+  res.set_content("q=" + q + ", limit=" + limit, "text/plain");
+});
+```
+
+`req.get_param_value()` pulls a value from the query string. Use `req.has_param("q")` if you want to check existence first.
+
+## Read request headers
+
+```cpp
+svr.Get("/me", [](const httplib::Request &req, httplib::Response &res) {
+  auto ua = req.get_header_value("User-Agent");
+  res.set_content("UA: " + ua, "text/plain");
+});
+```
+
+To add a response header, use `res.set_header("Name", "Value")`.
+
+> **Note:** `listen()` is a blocking call. To run it on a different thread, wrap it in `std::thread`. If you need non-blocking startup, see S18. Control Startup Order with `listen_after_bind`.
+
+> To use path parameters like `/users/:id`, see S03. Use Path Parameters.