Add npm as a system dependency in Getting Started guide.
Add npm as a system dependency in Getting Started guide.

With the recent django-pipeline related changes, npm is now a
required system level package.  This chage adds this requirement
to the docs.

Testing Done:
Built and viewed docs.

Reviewed at https://reviews.reviewboard.org/r/8024/

brennie/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,sgallagher/reviewboard,chipx86/reviewboard,chipx86/reviewboard,davidt/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,davidt/reviewboard,brennie/reviewboard,brennie/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,davidt/reviewboard,davidt/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,brennie/reviewboard,sgallagher/reviewboard