Adds a help center article that provides an overview and guide
for moving from a self-hosted Zulip organization to using Zulip
Cloud.
Updates the bullet points with the advantages of Zulip Cloud
that is also used in the help center article about choosing
between Zulip Cloud and self-hosting.
Fixes#24042.
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.
