docs: add C++ SDK reference page

pages/integrations/sdks/cpp.mdx

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+---
+title: C++ SDK
+navigation_icon: code
+---
+
+:::warning
+
+The C++ SDK is a work in progress.
+The API is subject to change and breaking changes may occur between releases.
+
+:::
+
+The Unikraft Cloud C++ SDK is an autogenerated client library based on the public OpenAPI specification.
+It uses [cpprestsdk](https://github.com/Microsoft/cpprestsdk) for HTTP and async operations via `PPL` tasks.
+
+## Dependencies
+
+| Dependency | Notes |
+|---|---|
+| [cpprestsdk](https://github.com/Microsoft/cpprestsdk) | HTTP client and `PPL` async runtime |
+| Boost (`boost-uuid`) | UUID support |
+| OpenSSL | TLS |
+| CMake >= 3.x | Build system |
+
+## Installation
+
+### Linux
+
+```bash title=""
+sudo apt-get install -y \
+  build-essential ca-certificates cmake \
+  libcpprest-dev libssl-dev make
+```
+
+### macOS
+
+```bash title=""
+brew install cpprestsdk
+```
+
+### Windows
+
+```bash title=""
+vcpkg install cpprestsdk cpprestsdk:x64-windows \
+  boost-uuid boost-uuid:x64-windows
+```
+
+## Build
+
+```bash title=""
+git clone https://github.com/unikraft-cloud/cpp-sdk
+cd cpp-sdk
+
+cmake \
+  -DCPPREST_ROOT=/usr \
+  -DCMAKE_CXX_FLAGS="-I/usr/local/opt/openssl/include" \
+  -DCMAKE_MODULE_LINKER_FLAGS="-L/usr/local/opt/openssl/lib"
+
+make
+```
+
+## Authentication
+
+Configure authentication manually via `ApiConfiguration`:
+
+```cpp title="main.cpp"
+#include <cstdlib>
+#include "UnikraftCloudV1APIClient/api/InstancesApi.h"
+
+using namespace cloud::unikraft::v1::api;
+using namespace cloud::unikraft::v1::model;
+
+int main() {
+    auto apiConfig = std::make_shared<ApiConfiguration>();
+    apiConfig->setBaseUrl("https://api.fra0.kraft.cloud/v1");
+    apiConfig->setApiKey(
+        "Authorization",
+        std::string("Bearer ") + std::getenv("UKC_TOKEN")
+    );
+
+    auto apiClient = std::make_shared<ApiClient>(apiConfig);
+    // ...
+}
+```
+
+The SDK has no automatic environment variable support.
+Pass the token explicitly.
+
+## Quickstart
+
+List all instances:
+
+```cpp title="main.cpp"
+#include <iostream>
+#include <cstdlib>
+#include "UnikraftCloudV1APIClient/api/InstancesApi.h"
+
+using namespace cloud::unikraft::v1::api;
+
+int main() {
+    auto apiConfig = std::make_shared<ApiConfiguration>();
+    apiConfig->setBaseUrl("https://api.fra0.kraft.cloud/v1");
+    apiConfig->setApiKey(
+        "Authorization",
+        std::string("Bearer ") + std::getenv("UKC_TOKEN")
+    );
+
+    auto apiClient = std::make_shared<ApiClient>(apiConfig);
+    InstancesApi instancesApi(apiClient);
+
+    instancesApi.getInstances({}, true)
+        .then([](pplx::task<std::shared_ptr<GetInstancesResponse>> task) {
+            auto resp = task.get();
+            if (resp && resp->getData()) {
+                for (auto& inst : resp->getData()->getInstances()) {
+                    std::cout << "Name:  " << inst->getName() << "\n";
+                    std::cout << "State: " << inst->getState() << "\n";
+                    std::cout << "---\n";
+                }
+            }
+        })
+        .wait();
+
+    return 0;
+}
+```
+
+## Resources
+
+| API class | Operations |
+|---|---|
+| `InstancesApi` | `createInstance`, `getInstances`, `getInstanceByUuid`, `startInstanceByUuid`, `stopInstanceByUuid`, `stopInstances`, `deleteInstanceByUuid`, `deleteInstances`, `getInstanceLogs`, `getInstanceLogsByUuid`, `getInstanceMetrics`, `getInstanceMetricsByUuid`, `waitForInstances` |
+| `VolumesApi` | Create, get, attach, detach, delete volumes |
+| `ServicesApi` | Create, get, delete service groups |
+| `AutoscaleApi` | Create, get, delete autoscale configurations and policies |
+| `CertificatesApi` | Create, get, delete certificates |
+| `ImagesApi` | Get images by tag or digest |
+| `UsersApi` | Get user quotas |
+
+All methods return `pplx::task<T>` for async execution.
+
+## Source
+
+The SDK source is available at
+[github.com/unikraft-cloud/cpp-sdk](https://github.com/unikraft-cloud/cpp-sdk).