Run Sphinx "make html" in a subshell.

sphinmake.zsh

#!/usr/bin/env zsh

# Begin Help block
my_name=$(basename "$0")
display_help() {
  cat <<EOF

*** Run Sphinx "make html" in a subshell.  ***
    usage: $my_name <No args are required>

EOF
}
[[ $1 == '--help' ]] && display_help && return 0
# End Help block

main() {
  (cd docs && make html && open build/index.html)
}

main "$@"