Adds l3 + l4 nav menus, with more intuitive scrolling to make wandering the
additional nav menu items much easier. Resolves#25. Resolves#76.
* Reuses some styles for l3 + l4 navs items
* Nav interaction was changed to be more intuitive:
* Nav is always sticky, doesn't scroll past end of content
* Nav scrolls up immediately on window scroll events
* Nav scrolls independently
* Adds TOC expand plus links to the left of nav elements with children, these
are added and update dynamically
* Links in nav don't automatically scroll nav
* Scrolling content will start scrolling nav again
* Adds URL fragment handling to link and show current anchor links
Reuses styling for wy-nav-vertical headers, which seem to be unused, with some
color changes to the header elements. Also fixes caption headerlink styling.
