SVG images not supported by PDF documentation

[Fabian Thiel]

By default there seems to be no support for svgs while building the pdf version of the documentation.

There seem to be workarounds here and here

As well as a direct solutions within sphinx: http://www.sphinx-doc.org/en/master/extdev/utils.html , but it seems that at the moment readthedocs does not support the sphinx.ext.imgconverter extension. https://github.com/rtfd/readthedocs.org/issues/3979

[Andreas Janz]

@Fabian Thiel I think we can ignore this problem and only support the HTML download.

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