[docs] Add C++ client documentation

website/docs/apis/client-support-matrix.md

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 title: "Client Support Matrix"
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 # Client Feature Support Matrix

website/docs/apis/cpp-client.md

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+---
+title: "C++ Client"
+sidebar_position: 4
+---
+
+# Fluss C++ Client
+
+The Fluss C++ Client provides a high-performance, synchronous interface for
+interacting with Fluss clusters. It manages an internal Tokio runtime and
+supports Apache Arrow for efficient data interchange.
+
+The client provides two main APIs:
+
+- **[Admin API](https://clients.fluss.apache.org/user-guide/cpp/api-reference#admin)**: For managing databases, tables, and partitions.
+- **[Table API](https://clients.fluss.apache.org/user-guide/cpp/api-reference#table)**: For reading and writing to Log and Primary Key tables.
+
+## Installation
+
+The C++ client is not yet published as a package and must be built from source.
+
+**Prerequisites:** CMake 3.22+, C++17 compiler, Rust 1.85+, Apache Arrow C++ library
+
+Install dependencies:
+```bash
+# macOS
+brew install cmake arrow
+
+# Ubuntu/Debian
+sudo apt-get install cmake libarrow-dev
+```
+```bash
+git clone https://github.com/apache/fluss-rust.git
+cd fluss-rust/bindings/cpp
+mkdir -p build && cd build
+cmake -DCMAKE_BUILD_TYPE=Release ..
+cmake --build .
+```
+
+For full build options including CMake integration into your own project, see the
+[C++ client installation guide](https://clients.fluss.apache.org/user-guide/cpp/installation).
+
+## Quick Example
+```cpp
+#include "fluss.hpp"
+
+int main() {
+    fluss::Configuration config;
+    config.bootstrap_servers = "127.0.0.1:9123";
+
+    fluss::Connection conn;
+    fluss::Result result = fluss::Connection::Create(config, conn);
+    if (!result.Ok()) {
+        std::cerr << "Connection failed: " << result.error_message << std::endl;
+        return 1;
+    }
+
+    fluss::Admin admin;
+    conn.GetAdmin(admin);
+
+    return 0;
+}
+```
+
+For more examples, see the [Fluss C++ Client documentation](https://clients.fluss.apache.org/user-guide/cpp/example/).
+
+## Full Documentation
+
+For the complete C++ client reference including all configuration options,
+API methods, data types, error handling, and worked examples — see the
+**[Fluss C++ Client documentation](https://clients.fluss.apache.org/user-guide/cpp/installation)**.