Wavedrom rendering broken in sonata documentation

The wavedrom renderer (see https://github.com/lowRISC/sonata-system/blob/main/util/mdbook_wavejson.py) used to generated images for register layouts seems to be broken in our current mdbook rendering setup. In particular see the SPI documentation: https://lowrisc.github.io/sonata-system/doc/ip/spi.html that just has the JSON that should be rendered to SVG.