Updates "help/manage-a-user" links that are contextually for general
information about users to go to the new "help/introduction-to-users"
article.
Adds a link to "Manage a user" in the related articles section of
the new intro to users article.
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.
