DOC: use new logo in the sphinx theme as well
With this change (https://github.com/pandas-dev/pandas-sphinx-theme/commit/84134e0804325538918324246f449e694466929b) in the theme, that should work.

However, locally, I am running into the problem that sphinx/html doesn't seem to like the svg logo's .. For local testing I added a converted png which works, but just replacing `.png` with `.svg` makes that the html does not include the image (at least in firefox).

cc @datapythonista 
