Make Latest/Stable Distinction Less Subtle in Documentation

This is increasingly a problem. (#1002, #1006)

Some ideas from #1002:

> I've been burned by this myself a number of times as it's quite subtle. I wonder if there's a theme or css customization we could make to make the stable/latest distinction louder. -- @ryneeverett

> As a note about the docs: another option would be making stable the default when you open, although probably a theme difference, or having the version in a more prominent place is also useful. -- @ferdinandyb