Previously, the help center has a broken link that just points to
Slack's help center. With this change, we will remove and replace the
link to their OAuth scope section.
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.
Anders Kaseorg
Joelute
David Rosa
Alya Abbott
Lauryn Menard