Resolve TODO in progress bar docs: add size use cases

Chad Lavimoniererequested to merge
duo/fix/3521-progress-bar-todos into main

What does this MR do?

Resolves the TODO placeholder in the Progress Bar component documentation's Sizes section.

The section listed three sizes (Extra small, Small, Medium) but had a TODO placeholder instead of guidance on when to use each. This MR adds use case descriptions for each size:

  • Extra small (4px): Dense/compact layouts like table rows or narrow sidebar widgets
  • Small (8px): Default size for most progress indicators in standard page content
  • Medium (16px): Primary focus of the view requiring greater visual prominence

The size values are derived from the component's Figma Code Connect mapping (xs=4px, sm=8px, md=16px).

Part of the TODO resolution effort tracked in #3521.

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