move logo images and similar files into an /assets folder #727

as of now, there are 3 image files for that MathCAT logo at the repo root. we should move them to a folder like /assets or similar, as projects typically do it from what I have seen out there.

only logo-inline.png is used in README as far as I can find it, so that should be simple.

and I just saw that there are duplicates in the /docs folder. is that a good way to handle this?