Navigation Tree View Examples: Add support for aria-selected for indicating currently displayed page

[mcking65]

In a navigation tree on a realistic site, the navigation tree exists on all pages included in the tree. So, when the new content or page is displayed, aria-selected indicates which node in the tree represents the current page and that node is in the tab sequence.

In addition, all nodes, including parent nodes, represent pages in the site. So, pressing enter or space on a parent node should activate that node, e.g., display a page associated with that node.

In order to simulate this, instead of opening the target page, click, enter, and space should make a change to a section of the page that displays to the right of the tree. In this section, it could just say something like "Pretend this is the place where the content of the page about "Oranges" or "apples" or whatever is displayed.

Of course, we should keep the wikipedia target URLs in place to demonstrate correct behaviors for opening a new tab or window.

[mcking65]

This issue is essentially duplicate of #1526. PR #1558 added aria-current to indicate the displayed page when it resolved #1526.