Setup samples validation in documentation via knit

[whyoleg]

Currently, samples are just code blocks, which could easily become outdated. At this moment, the easiest and most promising way for keeping those runnable and correct is to use https://github.com/Kotlin/kotlinx-knit
It parses Markdown documents, allows for linking to code, and extracts samples into runnable tests.
kotlinx.coroutines and kotlinx.serialization setup can be used as an example for the implementation.