The PDF documentation on readthedocs was missing the figures in the
fixtures reference chapter. This PR uses a Sphinx plugin that
automatically converts the checked-in SVG files to the PDF input files
that LaTeX requires.
The SVG-to-PDF conversion is done by inkscape, which gave the best
conversion among the tools I tried. However, it [does not yet
understand][href-bug] that you can write a plain `href` instead of
`xlink:href` in svg files, so I’ve had to edit the SVG files
accordingly.
[href-bug]: https://github.com/TeX-Live/luatex.git
e929d15848
