doc: add "What is Atom?" section

docs/.gitbook.yaml

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   summary: SUMMARY.md
 
 redirects:
-  core-concepts/what-is-atom: WIP.md
   core-concepts/atoms-and-modules: WIP.md
   core-concepts/lazy-evaluation-and-purity: WIP.md
   core-concepts/dependency-management: WIP.md

docs/core-concepts/what-is-atom.md

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+# What is Atom?
+
+Atom is a module system for Nix that transforms how we structure, share, and maintain Nix-based projects. It builds upon Nix's strengths to create a framework for truly modular, self-contained units of code.
+
+## Core Principles
+
+1. **Lazy Purity**: Extends Nix's lazy evaluation to file copying, ensuring efficiency without compromising purity.
+2. **True Modularity**: Self-contained units with clear boundaries and explicit interfaces.
+3. **Flexible Composition**: Free composition of Atoms, allowing versatile project structures.
+4. **Enhanced Sharing**: Private module member semantics for better encapsulation and safer code sharing.
+5. **Performance-Driven**: Engineered for efficiency, avoiding overhead common in other Nix modularity systems.
+6. **Focused Functionality**: Dedicated solely to being an excellent module system, excelling in its core purpose without feature bloat.
+
+## How Atom Transforms Nix Development
+
+1. **Efficient Resource Usage**: Copies files to the Nix store only when needed, reducing data transfer and storage.
+2. **Improved Code Organization**: Encourages cleaner, more maintainable code structures.
+3. **Enhanced Tooling Support**: Enables powerful static analysis and introspection without full Nix evaluation.
+4. **Simplified Dependency Management**: Offers granular control, reducing bloat and improving clarity.
+5. **Consistent Semantics**: Provides higher-level abstractions over Nix's low-level constructs, allowing developers to focus on code logic rather than Nix implementation details.
+
+## Who Benefits from Atom?
+
+- **Nix Beginners**: Lower entry barrier through clearer structure and improved tooling.
+- **Experienced Developers**: Enhanced modularity and efficiency for complex projects.
+- **Teams**: Improved code sharing and collaboration via well-defined module boundaries.
+- **NixOS Configuration Managers**: Simplified management of complex system configurations.
+- **Library Authors**: Easier creation and maintenance of reusable Nix code.
+
+## Atom vs. Traditional Nix Approaches
+
+| Feature                 | Traditional Nix | Flakes  | Atom |
+| ----------------------- | --------------- | ------- | ---- |
+| Lazy Evaluation         | ✓               | ✓       | ✓    |
+| Lazy File Copying       | ✓               | ✗       | ✓    |
+| Explicit Dependencies   | ✗               | ✓       | ✓    |
+| Private Module Members  | ✗               | ✗       | ✓    |
+| Static Analyzability    | ✗               | Partial | ✓    |
+| Unopinionated Structure | ✓               | Partial | ✓    |
+
+## Next Steps
+
+Explore Atom's core concepts:
+
+- [Atoms and Modules](./atoms-and-modules.md)
+- [Lazy Evaluation and Purity](./lazy-evaluation-and-purity.md)
+- [Dependency Management](./dependency-management.md)