Adds a shared note to both help center articles with sections on
subscribing users to streams, documenting that an automated private
message from the notification bot is sent to a user when subscribed
to a stream by another user.
Links to the article on the notification bot, and clarifies text in
that article regarding automated private messages.
Adds a new help center article focused on managing a single
user's stream subscriptions.
Creates a shared file for instructions to navigate to a user's
full profile via the right sidebar, which is used in three help
center articles.
Fixes#21795.