Add legend for package files sidenav

[sblom]

I had to go dig through the code to figure out what the squarish thing and hollow star were. (Sure, I could have solved that puzzle a different way, but y'know.)

So I decided to contribute a Legend block to help out other viewers in the same situation.

[praeclarum]

Oh defeat! Yeah the icons are pretty terrible. Thank you for this. I will be endeavoring to replace them with friendlier icons.