Run Pandoc commands in parallel in `docs/generate` (#332)
Run Pandoc commands in parallel in `docs/generate` (#332)

This speeds up the script a bit on my machine (140.5 ms ± 4.3 ms vs.
241.5 ms ± 6.0 ms as measured by
`hyperfine --warmup 10 ./docs/generate --shell=none`).
sharkdp/insect,sharkdp/insect,sharkdp/insect