Prepare 5.1.0 release

[aduth]

Changelog:

5.1.0

New Features

• Add support for SASS package entrypoints to limit or control loading of styles, similar to USWDS components. Available packages are packages/required, packages/global, packages/components, and packages/utilities.
• The Side Navigation component is now documented and includes updated and improved styling. While this should not require any markup changes, you should validate that the updated styling does not conflict with any product-specific styles for side navigation.

Bug Fixes

• Reduce the border width of the normal outline button to the intended width of 1px (previously 2px). The big variant of the outline button is unaffected by this change, and remains 2px.
• Fix rounded corners on sidebar current page item.
• Fix unintentional rounded corners on search text field.
• Fix utility classes not always applying as expected when applied to components. For example, using margin- utility classes on an unstyled button would previously unexpectedly conflict with and not take precedent over the button's own margins.
• Fix all instances of font weight appearance of unstyled buttons in content which applies anti-aliased font smoothing. Previous bug fix attempts had still missed default :hover and :active states.

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Visual regression failure is due to version text being shown and changed on the home page.