Improve Site Editor usability by adding descriptions under Design menu items

[dhruvikpatel18]

What?

This PR is intended to solve issue mentioned here: #55741

Why?

Many users, especially beginners, find it difficult to distinguish between templates and pages or understand where content originates. Adding descriptions provides contextual guidance, helping users navigate the Site Editor more efficiently and reducing confusion.

How?

• Updated the SidebarNavigationItem component to accept an optional description prop.
• Provided descriptive text for each menu item to clarify its purpose.
• Updated styles to remove default margins for the description to align with design guidelines.

Testing Instructions

1. Open the Site Editor in a block-based theme.
2. Navigate to the Design menu from the sidebar.
3. Verify that each item (Templates, Pages, Styles, Patterns, etc.) now includes a description.
4. Ensure descriptions provide meaningful guidance.
5. Test navigation by clicking on each menu item.

Screenshots or screencast

Before	After