Clarify duration spec

[saerdnaer]

During Camp2023 we came across an Event which took longer than 24h (due to a user input validation error) which broke the Android app as it was not cleary specified if 1:00:00 is H:MM:SS aka 1h or D:HH:MM aka 1day = 24h.

EventFahrplan/EventFahrplan#561

We could an a comment or validation to the XML + JSON schema to check for this issue. Normally the content of the duration field is ony HH:MM eh 1:15 for a 75 minute session

schedule/validator/json/schema.json

Lines 397 to 402 in e3bd7df

	"Duration": {
	"title": "Duration (hh:mm)",
	"type": "string",
	"examples": ["00:30", "01:30"],
	"pattern": "^[0-9]+:[0-9]{2}$"
	}

schedule/validator/xsd/schedule.xml.xsd

Lines 109 to 113 in e3bd7df

	<xs:simpleType name="duration">
	<xs:restriction base="xs:string">
	<xs:pattern value="([0-9]{1,2}:[0-9]{2})|([0-9]{1,2}:[0-9]{2}:[0-9]{1,2})"/>
	</xs:restriction>
	</xs:simpleType>