Tree view examples: Disable default focus ring and implement hover and focus states in css

[mcking65]

In the four tree view examples:

1. Disable default custom focus indicator
2. Per May 8, 2017 comment by Michiel in issue 223, instead of applying classes with js, use :hover and :focus to implement the custom focus and hover styling.
3. Update documentation in the accessibility features section of the example pages.

[jongund]

Removed default keyboard focus styling when focus is on a tree item

[mcking65]

With commits ba5cf02 and b5400fb, this issue is now resolved.