Use of Flexbox for the Hugo documentation

@digitalcraftsman Just now—on second thought—I’ve removed the highlighting from the top level menu item which contains the active document’s sub-item.

For example, regarding the image you captured, I’ve now removed the highlighting from “Getting Started.”

@jura Plus, just now—for the GitHub buttons—I’ve increased their vertical spacing when they wrap.

Also (just now), I’ve made the following, further improvements to the menu:

  • Increased the spacing (further);
  • Improved the font family; and
  • Increased the sub-item font size.

cc @rdwatters