Add user guide examples

docs/src/introduction.md

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+# Introduction
+
+Welcome to the **RustFrame User Guide**. This book provides a tour of
+RustFrame's capabilities from basic data handling to advanced machine learning
+workflows. Each chapter contains runnable snippets so you can follow along.
+
+To build this guide locally run `./build.sh` in the `docs/` directory. The
+chapters are arranged sequentially:
+
+1. [Data manipulation](./data-manipulation.md) for loading and transforming data.
+2. [Compute features](./compute.md) for statistics and analytics.
+3. [Machine learning](./machine-learning.md) for predictive models.
+4. [Utilities](./utilities.md) for supporting helpers and upcoming modules.
+
+Let's begin with some tabular data!