Update access API documentation and KDocs before release

[koperagen]

We now have a bit strange duplication:
https://kotlin.github.io/dataframe/apilevels.html - no mention of compiler plugin, page looks strange because it used to show 4 APIs and now it's 2 + example is not compatible with compiler plugin, only works in notebooks

Top level page with big banner on the top:
https://kotlin.github.io/dataframe/extensionpropertiesapi.html

I think these two should be somehow merged

In kdocs also no mention of compiler plugin here:
org.jetbrains.kotlinx.dataframe.documentation.AccessApi.ExtensionPropertiesApi

ApiLevels needs to be moved to samples module.

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Update access API documentation and KDocs before release

Motivation

Access API docs are currently inconsistent and partially outdated:

• apilevels.html still looks like an old overview page and does not mention the compiler plugin.
• Its example is not compatible with compiler-plugin usage and only works in notebook-style scenarios.
• extensionpropertiesapi.html is a separate top-level page with overlapping content.
• KDocs for AccessApi.ExtensionPropertiesApi also do not mention the compiler plugin.
• ApiLevels samples should be moved to the samples module.

Before release, users should see one clear story for typed column access: how extension properties work, when the compiler plugin is needed, and how this relates to the remaining access APIs.

Scope

• Merge or reorganize apilevels.html and extensionpropertiesapi.html to remove duplication.
• Update examples so they work for regular compiler-plugin usage, not only notebook scenarios.
• Mention the compiler plugin in relevant docs and KDocs.
• Move ApiLevels examples to the samples module.

Definition of Done

• Access API docs have no duplicated or contradictory top-level pages.
• Compiler plugin is clearly documented as the main way to use generated extension properties.
• Examples compile in the intended non-notebook setup.
• AccessApi.ExtensionPropertiesApi KDocs are updated.
• ApiLevels samples are moved and validated through the docs/sample pipeline.