tools: doc: improve async workflow of generate.js

Use fs.promises for read and write file
Use unified().process wich is async instead processSync
html and json are write in parallel
errors are logged and exit process with `1` code

Fixes: https://github.com/nodejs/node/issues/30090

PR-URL: https://github.com/nodejs/node/pull/30106


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>