[FEATURE] Add checkbox component

[albx]

Implement a BitCheckbox component inspired by the design and UX of the Bootstrap Italia Checkbox component.

Requirements:

• Create a reusable BitCheckbox component following Bootstrap Italia's Checkbox guidelines (docs).
• Support both single checkbox and checkbox group scenarios.
• Bind checked/unchecked state to a data model (including lists for groups).
• Expose properties for label, help text, hint, and validation messages.
• Support for indeterminate state and custom values.
• Allow disabled and read-only states.
• Provide custom styling and templates for label and checkbox.
• Ensure full accessibility including keyboard navigation, ARIA attributes, and screen reader support.
• Responsive design and mobile-friendly behavior.
• Show error/success messages following Bootstrap Italia validation standards.

Acceptance Criteria:

• BitCheckbox visually and functionally matches Bootstrap Italia Checkbox as closely as possible in Blazor.
• Component supports both single and group use cases, including model binding.
• All properties (label, hint, help text, disabled/read-only, indeterminate, validation) are implemented and documented.
• Accessibility and keyboard navigation requirements are fully met.
• Responsive layout and mobile support are present and demonstrated.
• Validation feedback is shown as per Bootstrap Italia guidelines.
• Example usage and documentation available in the repository and demo site.

References

• Bootstrap Italia Checkbox: https://italia.github.io/bootstrap-italia/docs/form/checkbox/