Adds a section to "View Zulip version" article for viewing the
mobile app version, and adds the mobile app installation guide
to the related articles section.
Part of #34748.
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.
