docs: add README badges and API reference link

Add CI status, C++23, header-only, platform, and documentation
badges to README. Add navigation bar linking API Reference (GitHub
Pages), Usage Guide, and Releases below the badge row.

Author: dasmig

README.md

@@ -5,8 +5,15 @@
 [![Entity Generator for C++](https://raw.githubusercontent.com/dasmig/entity-generator/master/doc/entity-generator.png)](https://github.com/dasmig/entity-generator/releases)
 
 [![GitHub license](https://img.shields.io/badge/license-MIT-blue.svg)](https://raw.githubusercontent.com/dasmig/entity-generator/main/LICENSE)
+[![CI](https://github.com/dasmig/entity-generator/actions/workflows/ci.yml/badge.svg)](https://github.com/dasmig/entity-generator/actions/workflows/ci.yml)
 [![GitHub Releases](https://img.shields.io/github/release/dasmig/entity-generator.svg)](https://github.com/dasmig/entity-generator/releases)
 [![GitHub Issues](https://img.shields.io/github/issues/dasmig/entity-generator.svg)](https://github.com/dasmig/entity-generator/issues)
+[![C++23](https://img.shields.io/badge/standard-C%2B%2B23-blue.svg)](https://en.cppreference.com/w/cpp/23)
+[![Header-only](https://img.shields.io/badge/type-header--only-green.svg)](#integration)
+[![Platform](https://img.shields.io/badge/platform-linux%20%7C%20windows%20%7C%20macos-lightgrey.svg)](#)
+[![Documentation](https://img.shields.io/badge/docs-GitHub%20Pages-blue.svg)](https://dasmig.github.io/entity-generator/)
+
+**[API Reference](https://dasmig.github.io/entity-generator/)** · **[Usage Guide](doc/usage.md)** · **[Releases](https://github.com/dasmig/entity-generator/releases)**
 
 ## Features