Scheduler: describe Hidden Week Days demo (DevExpress#33418)

Signed-off-by: Vlada Skorokhodova <94827090+vladaskorohodova@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Elena Khamlyuk <80813840+khamlyuk@users.noreply.github.com>

Author: 3 people

apps/demos/Demos/Scheduler/HiddenDays/description.md

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+The DevExtreme Scheduler allows you to exclude specific days of the week (using the [hiddenWeekDays](/Documentation/ApiReference/UI_Components/dxScheduler/Configuration/#hiddenWeekDays) property).
+
+Use checkboxes in the options panel to toggle day visibility. A validation message appears if you attempt to hide all seven days, since at least one day must remain visible.
+<!--split-->
+
+To hide specific days of the week, assign an array of day indexes to the [hiddenWeekDays](/Documentation/ApiReference/UI_Components/dxScheduler/Configuration/#hiddenWeekDays) property. The index values follow the JavaScript `date.getDay()` convention:
+
+- `0` - Sunday
+- `1` - Monday
+- `2` - Tuesday
+- `3` - Wednesday
+- `4` - Thursday
+- `5` - Friday
+- `6` - Saturday
+
+The property works across multiple view types. To hide days of the week from a specific view, use the [same property](/Documentation/ApiReference/UI_Components/dxScheduler/Configuration/views/#hiddenWeekDays) within the view configuration object.