Center the section headers

In my opinion this looks better in line with the other centered content.
This commit can safely be reverted if the look & feel is not right.
Fix myths section width on mobile

It was too wide causing a horizontal scroll to appear.
Try another stab at myth colors

Still not looking good, but I can’t seem to get it much better now.
Reduce the height of the mobile nav menu
Hide the mobile nav menu on click
Fix the multiple highlighted nav menu elements bug

When page is scrolled down and then refreshed, the initialization code
of the waypoints menu was highlighting multiple elements as active since
the waypoint() callback triggers for each menu element past which the
browser has scrolled on load.