Add "no gradient" logos used for astropy fashion and fix a few fonts

I added a couple logos where the icon has a sold color instead of a gradient. These (or some similar version) were used for the astropy clothing.

At the same time I noticed that a few of the font weights were wrong in some of the logos, at least when viewed with inkscape on my machine. So I fixed those ("astro" is semi-bold and "py" is light).

Finally I added a table to the README that describes the contents of the files, as it is not always clear from the names.