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

Checklist

• vcbuild test (Windows) passes
• commit message follows commit guidelines

I'm behind a corporate proxy and get some network errors during the tests, so I hope tests passes with the CI.