docs(prerequisites): update Node.js prerequisites documentation

This change updates the version of the Node Version Manager suggested by
the prerequisites documentation. The NVM installation command line hint
has been replaced with the snippet provided by NVM's user guide, and the
second line now automatically installs a version of Node.js compatible
with TF-A's repository scripts.

Change-Id: I6ef5e504118238716ceb45a52083450c424c5d20
Signed-off-by: Chris Kay <chris.kay@arm.com>