docs: Create a new tutorial for building a functional component

The "Describing a View" guide introduces users to the two component models in Neo.mjs. A logical next step is a hands-on tutorial that walks them through creating their own component using the modern, functional approach.

This ticket is to create a new tutorial, "Creating a Custom Functional Button", that serves as a practical, step-by-step guide for new users.

Tasks

• Create a new tutorial file at learn/tutorials/CreatingAFunctionalButton.md.
• The tutorial should guide the user through the following steps:
  1. Defining a basic functional component with defineComponent.
  2. Adding reactive public configs (e.g., text_, iconCls_).
  3. Using the configs within the createVdom method to build a dynamic VDOM.
  4. Implementing event handling by adding a handler_ and an onClick method.
  5. Providing a complete, live-preview example that demonstrates how to use the newly created custom component within a view.
• The introduction must clarify that Neo.mjs already has a feature-rich functional button (Neo.functional.button.Base) and that this tutorial is for educational purposes to teach the core concepts of functional component creation.
• Update learn/tree.json to add the new tutorial to the "Tutorials" section, making it discoverable in the navigation tree.