Add Cookbook S01-S22 (draft)

docs-src/pages/en/cookbook/s21-thread-pool.md

@@ -0,0 +1,69 @@
+---
+title: "S21. Configure the Thread Pool"
+order: 40
+status: "draft"
+---
+
+cpp-httplib serves requests from a thread pool. By default, the base thread count is the greater of `std::thread::hardware_concurrency() - 1` and `8`, and it can scale up dynamically to 4× that. To set thread counts explicitly, provide your own factory via `new_task_queue`.
+
+## Set thread counts
+
+```cpp
+httplib::Server svr;
+
+svr.new_task_queue = [] {
+  return new httplib::ThreadPool(/*base_threads=*/8, /*max_threads=*/64);
+};
+
+svr.listen("0.0.0.0", 8080);
+```
+
+The factory is a lambda returning a `TaskQueue*`. Pass `base_threads` and `max_threads` to `ThreadPool` and the pool scales between them based on load. Idle threads exit after a timeout (3 seconds by default).
+
+## Also cap the queue
+
+The pending queue can eat memory if it grows unchecked. You can cap it too.
+
+```cpp
+svr.new_task_queue = [] {
+  return new httplib::ThreadPool(
+    /*base_threads=*/12,
+    /*max_threads=*/0,   // disable dynamic scaling
+    /*max_queued_requests=*/18);
+};
+```
+
+`max_threads=0` disables dynamic scaling — you get a fixed `base_threads`. Requests that don't fit in `max_queued_requests` are rejected.
+
+## Use your own thread pool
+
+You can plug in a fully custom thread pool by subclassing `TaskQueue` and returning it from the factory.
+
+```cpp
+class MyTaskQueue : public httplib::TaskQueue {
+public:
+  MyTaskQueue(size_t n) { pool_.start_with_thread_count(n); }
+  bool enqueue(std::function<void()> fn) override { return pool_.post(std::move(fn)); }
+  void shutdown() override { pool_.shutdown(); }
+
+private:
+  MyThreadPool pool_;
+};
+
+svr.new_task_queue = [] { return new MyTaskQueue(12); };
+```
+
+Handy when you already have a thread pool in your project and want to keep thread management unified.
+
+## Compile-time tuning
+
+You can set the defaults with macros if you want compile-time configuration.
+
+```cpp
+#define CPPHTTPLIB_THREAD_POOL_COUNT 16       // base thread count
+#define CPPHTTPLIB_THREAD_POOL_MAX_COUNT 128   // max thread count
+#define CPPHTTPLIB_THREAD_POOL_IDLE_TIMEOUT 5  // seconds before idle threads exit
+#include <httplib.h>
+```
+
+> **Note:** A WebSocket connection holds a worker thread for its entire lifetime. For lots of simultaneous WebSocket connections, enable dynamic scaling (e.g. `ThreadPool(8, 64)`).