The instructions for the desktop and mobile apps for this section
are for when the user has already logged into an initial Zulip
organization, so we update the section header to be "Log in to a
new organization" instead of "Log in for the first time".
For both apps, if the user has not logged in to any Zulip
organization, then the apps default to the log in prompts, so
there is no need to add a set of instructions for that case.
Follow-up to feedback in #34869.
Documents the Do Not Disturb option with a dedicated page in
the Notifications section.
The capitalization is not our standard style because the desktop app
doesn't follow that style convention.
Fixes#24169.