Latexpdf build broken - missing dependency

It looks like `librsvg2-bin` must be installed in order to create the latex pdf files. I did a `pip install --user -e .[docs]`, but that didn't install all dependencies (`sphinxcontrib-svg2pdfconverter` got installed). Maybe this can't be automated, but at least it needs to be documented somewhere. On Ubuntu I needed to do `sudo apt-get install librsvg2-bin`