Improve Nav menu behavior

[RichieEscarez]

Add click to close nav section:

Allow users to click on a menu section (ie. a topic in the menu that include child nodes) to not only open but also close. For sections with lots of child nodes that span the whole screen, its better (expected) behavior to click once more to close that really long section (to view the contents that gets pushed off the screen).

Investigate Istio CSS: https://istio.io/docs/
(They use icons to indicate a "section", which you can click to open/close those sub-trees)