redirect legacy scatter plot matrix doc

[cldougl]

It's pretty confusing to have both of these doc pages next to each other:

• We should move the original figure factory splom page to /legacy (in repo see _posts/python/legacy/)(https://plot.ly/python/legacy/scatterplot-matrix/) as was done with the violin plots:

• and redirect https://plot.ly/python/scatterplot-matrix/ to https://plot.ly/python/splom/

• change this title to Scatterplot Matrix
  

• remove the legacy doc from the statistical thumbnail display