Update access API documentation and KDocs before release

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Access API docs are currently inconsistent and partially outdated: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.

Start by comparing apilevels.html and extensionpropertiesapi.html, then read the AccessApi.ExtensionPropertiesApi KDocs and locate the ApiLevels samples. Reorganize the overlapping documentation, document compiler-plugin usage, update the examples for non-notebook use, and move ApiLevels to the samples module. Validate the examples and documentation through the docs/sample pipeline.

Written by the indexing model from the issue text.