[FEATURE] Add support for avatar stacked group element

Implement a BitAvatarGroup component inspired by the design and UX of the Bootstrap Italia Avatar Group List component.

> [!IMPORTANT]
> This component requires Dropdown component

### Requirements:
Describe the list of requirements this feature needs:
- Create a reusable BitAvatarStackedGroup component following Bootstrap Italia's guidelines ([docs](https://italia.github.io/bootstrap-italia/docs/componenti/avatar/#gruppi-di-avatar)).
- Support groups with textual, images and link avatar
- Support color variants (primary, secondary, success, danger, warning, info, light, dark, etc.).
- Support small and medium avatar sizes
- Responsive design and mobile-friendly behavior.
- Requires #46

### Acceptance Criteria:
- The component renders valid Bootstrap Italia avatar markup.
- All documented variants and styles are supported via parameters.
- The component is unit tested for all major styles.
- Usage examples are provided in the documentation or a sample page.

### References
- Bootstrap Italia - List of Avatars documentation ([docs](https://italia.github.io/bootstrap-italia/docs/componenti/avatar/#gruppi-di-avatar))