Add comprehensive examples for Menu, MenuItem, MenuEditor, and MenuPagination

[Copilot]

Added detailed code examples to README.md covering all core MenuAPI concepts.

Examples Added

• Basic Menu: Simple menu creation and player opening
• MenuItem: Three interaction patterns (close, teleport, item-giving)
• MenuEditor: Custom dynamic content editor displaying player stats
• MenuPagination: Three layout patterns (full, middle, custom slots)

Examples progress from basic to advanced, demonstrating practical use cases:

// Simple interactive button
setSlot(13, new MenuItem(teleportItem) {
    @Override
    public void click(Click click) {
        click.player().teleport(spawn);
        click.player().closeInventory();
    }
});

// Custom editor for dynamic content
public class PlayerStatsEditor implements MenuEditor {
    @Override
    public void edit(@NotNull Menu menu, @NotNull Inventory inventory) {
        // Updates menu with live player health, food, XP
    }
}

// Pagination with custom slot layout
MenuPagination pagination = MenuPagination.create(
    39, 41, PAGE_CHANGER, customSlots
);

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