Improve project documentation

[Copilot]

The project lacked onboarding clarity: no learning objectives, no project structure map, no English workshop guide, and zero inline code documentation across all Dart source files.

Changes

README.md

• Added "What you'll learn" section listing the six Flutter concepts covered
• Added annotated project structure directory tree
• Added "Running the app" quick-start commands (flutter pub get / flutter run) and platform-expansion snippets
• Linked to docs/guide.md from the hands-on-lab section; added no-Git download tip

docs/guide.md

• Added full English translation of the previously Spanish-only guide
• Expanded with a 6-step walkthrough including tables (what to create, where, with which fields)
• Original Spanish content preserved below a clear separator

app/README.md

• Replaced single-line placeholder with a brief description and links to root README and guide

Dart source files — docstrings only, no logic changed

Added /// docstrings to every public class, field, and method across all source files:

/// Lightweight in-memory state container for the CoffeeShop app.
///
/// Holds all mutable application data and exposes simple methods to
/// manipulate it. A single instance is shared via [CoffeeShopApp.manager].
///
/// > **Note:** No state management library; not for production use.
class CoffeeManager { ... }

/// Converts the current cart into a new [Order], appends it to [orders],
/// and clears the cart. Does nothing if the cart is empty.
void makeOrder() { ... }

Files covered: main.dart, coffee_manager.dart, all three models, all four pages.

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