Move Writerside Korro samples out of `:core` and standardize them on `SampleHelper`

[Jolanrensen]

Problem

Some code samples inserted into Writerside documentation via Korro are still located in :core.

This is undesirable because :core should contain library code and tests, not Writerside documentation samples. Documentation samples also currently use older infrastructure such as DFExplainer, which makes them harder to maintain and may require internal access.

Expected

Use :samples as the unified source for Writerside samples and generated documentation examples.

Scope

• Remove from :core all code samples inserted into Writerside sources using Korro
• Move/rewrite those samples in :samples
• Deprecate DFExplainer in favor of SampleHelper
• Rewrite existing relevant samples with SampleHelper
• Keep :core tests focused on library tests, not Writerside samples
• KDoc-specific samples may be handled separately later

Acceptance criteria

• No Writerside/Korro documentation samples remain in :core
• :samples is the unified source for Writerside samples
• Existing Writerside sample generation still works
• Kandy plot / DataFrame iframe samples continue to be handled from :samples
• Existing DFExplainer usages are migrated to SampleHelper where applicable
• Any remaining :core samples are explicitly limited to tests or future KDoc-specific needs

Motivation

Documentation samples should be maintained as real user-facing examples, not as internal :core test code.

Moving them to :samples reduces coupling to internals, makes Korro-based documentation easier to maintain, and helps keep examples reliable before 1.0.

Initial comment from @Jolanrensen

Code samples for the documentation website reside in [core/.../test/.../samples/api](./core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/samples/api),
and [tests/.../samples/api](./tests/src/test/kotlin/org/jetbrains/kotlinx/dataframe/samples/api)
and they are copied over to Markdown files in [docs/StardustDocs/topics](./docs/StardustDocs/topics)
by [Korro](https://github.com/devcrocod/korro).

[AndreiKingsley]

So. I tried different things.

1. "tests" module can no be moved into the "core/test" - it depends on Kandy which requires jvm >=11.
2. "samples" from "core/test" are really hard to move to the "tests" module - there're a lot of internal staff.

So my solution is simple.

1. Keep "tests" module (maybe rename it to "samples") and use it for creating DF and Kandy plot samples for docs.
2. After migration to new HTML dataframes in docs, we can remove old "samples" tests (except for those used by Kodex in KDocs.).

[Jolanrensen]

"tests" module can no be moved into the "core/test" - it depends on Kandy which requires jvm >=11.

Agree, not only that, but it depends on other modules too.

"samples" from "core/test" are really hard to move to the "tests" module - there're a lot of internal stuff.

Friend module? :))

But yes, sounds like a plan!

[AndreiKingsley]

Friend module? :))

I tried, but there're a lot of issues with cross-dependencies, so it would take too much time to resolve all of them.

[AndreiKingsley]

I would reformulate what needs to be done within this issue as follows:

1. Remove from :core all code samples that are inserted into Writerside sources using Korro.
   • we can then make special samples for KDocs in future
2. Deprecate DFExplainer in favor of SampleHelper and rewrite all existing samples with SampleHelper.

:samples (former :tests) module now serves as unified WS samples source and kandy-plot/df-iframe samples handler.
:core tests should not contain WS samples (but might be for Kdocs)

@Jolanrensen do you agree?