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.
This commit adds a note to NOT shut down Rocket.Chat server
while creating database dump using mongodump utility because
as this results in bugs like empty instances.bson file and
empty uploaded data after import.
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.