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.
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.
David Rosa
Alya Abbott
Lauryn Menard