[scheduler] Timezone Issue 9: Documentation, demos, and final clean-up for timezone support

[rita-codes]

• Document the new timezone prop, including the semantics of 'default' and TemporalTimezone.
• Clarify that events can have different original timezones and that the Scheduler renders everything in the chosen render timezone while keeping original timezones in the data.
• Add at least one demo using a non-default timezone and, optionally, a multi-timezone events example.
• Add regression tests for event creation (using 'default') and verify there are no leftover ad-hoc timezone conversions that duplicate the new centralized logic.