style guide nav update
style guide nav update

• add focusable=“false” to svg icon in menu button
• add `aria-label=“style guide”` to primary style guide nav, to increase it’s discoverability as the primary nav, when other nav examples exist on the page
• remove href attribute from menu-toggle `<button>`

cehfisher/a11y-style-guide,cehfisher/a11y-style-guide