In the help center and production documentation, replace links to
the "Getting your organization started with Zulip" guide to the
new "Moving to Zulip" guide.
These files are not Jinja2 templates, so there's no reason that they needed
to be inside `templates/zerver`. Moving them to the top level reflects their
importance and also makes it feel nicer to work on editing the help center content,
without it being unnecessary buried deep in the codebase.
