[Site Admin]: Implement SiteHub Component

[retrofox]

Introduce the SiteHub component in the Site Admin package, following WordPress Core's implementation.
This component will serve as a central UI element, providing quick access to site navigation, the command palette, and key actions related to the current site.

The component should integrate with @wordpress/core-data to retrieve the site title and home URL while ensuring compatibility with the existing routing system.

It should include a SiteIcon, provide a button to navigate back to the dashboard, and support opening the command palette. The structure should align with Core’s approach while maintaining flexibility for customization within Site Admin.

[github-actions]

OpenAI suggested the following labels for this issue:

• [Feature Group] Site Navigation: The issue discusses implementing a central UI element for site navigation, indicating the focus on improving site navigation features.
• [Feature] Action Bar: The introduction of a SiteHub component with navigation and command palette functionality aligns with enhancing the action bar's capabilities.
• [Feature] Site Settings: The component aims to integrate with core data for retrieving site-specific information, which relates to the management of site settings.