Page component - aria-expanded attribute for button that shows primary nav

[jgiardino]

When a button toggles the display of contents inline (i.e. disclosure), we should include additional cues that communicate the state of the expanded contents as either expanded or collapsed

The wai-aria Disclosure APG recommends using aria-expanded to indicate to assistive technology what the current state is. For example, in our core Table example, a similar button is announced as “Node 1 Details button expanded.”

Updates for this issue include:

• Add the attribute aria-expanded to the button labeled "Toggle primary navigation"

• Add this attribute to the accessibility documentation, with an explanation on it's usage. For example:

  Attribute	Applied To	Outcome
  aria-expanded="true/false"	.pf-c-page__header-brand-toggle > .pf-c-button	Indicates that the expandable content is visible and the current state of the contents. Required