Utils: add page about DateTime

[f3l1x]

• DateTime::from
• DateTime::fromParts
• DateTime::modifyClone
• DateTime::setTimestamp
• DateTime::getTimestamp
• DateTime::createFromFormat
• DateTime::jsonSerialize

---

https://api.nette.org/2.4/Nette.Utils.DateTime.html

[f3l1x]

Already added, thanks @Barabrossa.