Add datetime parsing/converting guide-testcase

#1777 introduced A LOT of new regarding datetime — methods and (most important) new implementation;
The API is extensive, with many nuances and features — it’s easy to get lost in all of it, but it’s an extremely important part!

This deserves its own guide on the documentation website, which will provide a detailed description of all the features and specifics for Java/Kotlin, etc.

Also important for #1802.

This guide will also serve as a great test as well!

Start by reviewing the datetime changes introduced in #1777 and the related work in #1802. Map the API's methods and nuances into a guide for the documentation website, and define the accompanying testcase so the documented Java/Kotlin behavior is covered.

Written by the indexing model from the issue text.