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.
Renames file and redirects existing links to "help/user-roles" as
the previous content tracks to that current file's content.
Updates existing links (excluding ReadtheDocs) to go to either
"help/user-roles" or "help/manage-permissions" depending on which
help center article applies to the content.
This commit rename the existing setting `Who can invite users to this
organization` to `Who can send email invitations to new users` and
also renames all the variables related to this setting that do not
require a change to the API.
This was done for better code readability as a new setting
`Who can create invite links` will be added in future commits.
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.
