Make nodejs version requirement consistent across documentation

There are several places where nodejs is required to be installed in the documentation as it is used for building nuxeo and using the cli.

However, these versions are often different from one page to another. Here are examples I've found, there might be others.

• use the latest stable (https://doc.nuxeo.com/nxdoc/develop-with-nuxeo-platform/#prerequisites)
• use 6.4.0 (https://doc.nuxeo.com/corg/installing-nodejs-and-related-tools/)
• use 0.10.32 (https://github.com/nuxeo/nuxeo/blob/master/README.md#requirements)
• use 6.5.0 min (https://doc.nuxeo.com/nxdoc/setting-up-your-javascript-environment/#setting-up-the-javascript-environment)

For information, current stable version of nodejs is 8.9.1 (https://github.com/nodejs/Release#release-schedule)