[FEATURE] Add the Modal component

[albx]

Create a reusable Razor component for modals, following the styles and guidelines in Bootstrap Italia - Modal. Support all modal variants, sizes, and accessibility requirements.

Requirements:

• Implement a Razor component (e.g., BitModal.razor) that encapsulates Bootstrap Italia modal styles.
• Support for all documented modal sizes and variants.
• Allow for custom header, body, and footer templates.
• Expose parameters for ARIA/accessibility attributes and backdrop behavior.
• Add XML documentation for all public parameters.

Acceptance Criteria:

• The component renders valid Bootstrap Italia modal markup.
• All documented features and variants are supported via parameters.
• The component is unit tested for major variants and use cases.
• Usage examples are provided in the documentation or a sample page.

References:

• Bootstrap Italia - Modal Documentation