Button and Placeholder API updates

Refined Button and Placeholder APIs with a smaller set of button variants.

Improved

Updated the Button and Placeholder component APIs and their usage documentation.

The release narrows the public API around the treatments that are supported and documented, reducing ambiguous variants in application code. Placeholder guidance was updated at the same time so loading states use the same composition patterns as the rest of the component library.

Breaking changes

Button no longer supports variant="plain". Review those instances and choose a supported primary, secondary, or tertiary treatment.