Add legend explaining symbol usage

From Gos:
> would be good to have a key to explain that circles are GO terms and squares are Genes
(if that is what is meant); that colours are assigned randomly; that wedges of colour in
the GO term symbols helps indicate how many connected genes there are. (This is assuming
I've guessed right).

Review the interaction-network visualization and its existing symbol and color rendering first; no file or test is named in the issue. Done means a visible legend explains circles versus squares, random colors, and the wedges indicating connected-gene counts, with those interpretations confirmed.

Written by the indexing model from the issue text.