Improve DateTimeType examples

[jlaur]

This is now possible after openhab/openhab-core#5481.

This is better than the previously provided examples, because it uses the currently configured time zone in openHAB rather than the JVM time zone.

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[dilyanpalauzov]

Now I think a comment in the first example would be useful, stating from which object which method does the example calls. This will help the users to understand, what is going on.

[dilyanpalauzov]

E.g.

ZoneId refers to class java.time.ZoneId, zoneId calls method ScriptServiceUtil.getZoneId().

[dilyanpalauzov]

Source code of the above:

`ZoneId` refers to class [java.time.ZoneId](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/time/ZoneId.html), `zoneId` calls method [ScriptServiceUtil.getZoneId()](https://www.openhab.org/javadoc/latest/org/openhab/core/model/script/scriptserviceutil#getZoneId()).