Adds the missing aria-controls, aria-expanded and - because of the use of a single unicode character on the toggle, with no actual content that can be read by assistive tech - aria-label to the navbar collapsible content examples.
Moves the first collapsible content after the toggle for accessibility/focus and reading order issues.
Modifies the site-wide main navigation to follow the collapsible content navbar pattern from the actual documentation.
Note: because v4's current styles for collapsible navbars are a bit...broken, this also introduces the same brokenness to the site-wide navigation. Even more reason to tackle this soon...