Add NodeJS as prerequisite

[bertocq]

As noted by @aitbw at consuldemocracy/consuldemocracy#2486 we should include NodeJS as a requirement on all /getting_started/prerequisites guides.

[aitbw]

One step ahead of you, @bertocq, I'm already working on it updating both English and Spanish docs accordingly 😄

Just one thing I'd like to discuss: Preferred method to install Node.js? I'm currently using tj/n and works flawlessly but it'd be better if we could discuss it first 😌

[bertocq]

Awesome @aitbw 👏. As you know I'm not very up to date with JS environment, and just use nvm because someone said so somewhere... but whatever the nodejs community has as a good standard would do :).

I think both rbenv and rvm are okey at Ruby community, but we're explicitly mentioning only rbenv at macos documentation because thats the combination we all use :( Maybe we should be more clear anything that installs the right version of ruby would be 👌 ... and same should be applied to NodeJS 🤔 ... but anything would be far better than what we have right now (no mention whatsoever to nodejs 😞 )

Thanks!