Add Node.js requirement to readme

[bmorelli25]

On my new laptop I was unable to build after installing .NET:

➜  docs-builder git:(fix/no-auto-newline) dotnet publish "src/docs-builder/docs-builder.csproj"                                             

Restore complete (0.8s)
  Elastic.Markdown failed with 1 error(s) (0.1s)
    /Users/brandonmorelli/Documents/github/docs-builder/src/Elastic.Markdown/Elastic.Markdown.csproj(25,5): error MSB3073: The command "npm ci" exited with code 127.

I had to also install Node.js and NPM. I'm not sure which is the actual requirement so creating an issue.

To be updated:
https://github.com/elastic/docs-builder?tab=readme-ov-file#run-without-docker