ReadTheDocs build failing for pdflatex

[flongford]

It looks like the latest merge to master (#19) caused a readthedocs build failure.

From the build logs it looks like the HTML files seem to have been generated fine, but the pdf files were not: https://readthedocs.org/projects/force-tutorial/builds/11159119/

Some errors report problems handling .SVG files, as well as broken hyperlinks

[flongford]

We can leave out the PDF build for now, if we don't intend to publish the material offline.