Spike: test stdlib's API reference built by modern Dokka

[IgnatBeresnev]

Once stdlib is updated to the latest version of Dokka (#3535), it'd be nice to test it before it's released.

However, because stdlib has never been tested by the Dokka team before, it's difficult to estimate how long the actual testing will take due to many unknowns:

• How to build documentation for stdlib with Dokka? (locally / CI)
• What's the difference between stdlib and any other kotlinx library? (built-ins, special features like sinceKotlin and all types, runnable samples, versioning, etc)
• What should a test plan look like?

These and any other arising questions can be researched before #3535 is actually implemented, so as to prepare for testing in advance. This issue exists to keep track of it.

Ideally, the outcome should be either a new issue with the information that was researched or some sort of a document / article with the same information.