doc additions

I looked through the documentation a bit to see what I was missing initially to understand all the groups and batches in the connection. The main thing that helped me was just constructing a simple 1D refinement connection and going through the various from_element_indices, etc. to see what they point to. I could write something about that in more detail, but just reading the code and playing in a debugger did the trick for me pretty fast, so I don't know if it would be that useful after all..

I did center some images, so this isn't completely useless!