Instead of referring to users being added or removed from channels,
we now use subscribe or unsubscribed from channels.
Splits the article for adding and removing users from a channel
into separate articles: /help/subscribe-users-to-a-channel and
/help/unsubscribe-users-from-a-channel.
The URL redirects for the combined add/remove articles (for both
channel and stream terminology) go to the subscribe users to a
channel 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.
Fixes#31286.
While our current implementation of help center renders the table
without those three dashes, our new help-beta using starlight needs
those 3 dashes there.
Before, the `span` opening tag was split between two lines. This
led to `[@mdx-js/rollup]` throwing an error on the build step saying
`Expected a closing tag for <span> before the end of paragraph`.
This is a preparatory commit for the migration to use starlight
for help center docs.
