docs: write docs, add README

docs/index.md

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 # Rensa Core
+
+Welcome to the documentation for **Rensa**, a powerful and flexible Nix flake framework.
+
+Rensa is designed to help you organize your Nix projects into logical units called **Cells**,
+making your code more modular, maintainable, and scalable.
+
+## Getting Started
+
+If you are new to Rensa, we recommend starting with the [Tutorial](tutorial.md) to build your first project.
+
+## Core Concepts
+
+Understand the philosophy behind Rensa by reading about [Concepts](concepts.md), including:
+
+- **Cells**: Functional domains of your project.
+- **Blocks**: Typed outputs within cells.
+- **Actions**: CLI commands for your cells.
+
+## API Reference
+
+For detailed information on Rensa's functions and helpers, check the [API Reference](api.md).
+
+## Ecosystem
+
+See [Related Libraries](libraries.md) to see how to extend Rensa with pre-built solutions for development shells, documentation, CI, and more.
+
+## Further Reading
+
+Rensa is inspired by `std`. For more in-depth information on the underlying concepts, you can refer to the [std Reference](https://std.divnix.com/reference/std.html).