Add KDocs for function `Cast` and `CastTo`

[zaleslaw]

Add KDocs, tests, and better site examples for cast / castTo

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/cast.kt contains public cast / castTo APIs, but their documentation and tests are uneven.

The site page exists: https://kotlin.github.io/dataframe/cast.html, but examples are minimal and do not clearly cover the main use cases.

Scope

• Add or improve KDocs for public cast / castTo overloads in cast.kt.
• Make KDocs clear about the difference between:
  • unchecked cast-like APIs that only change the formal type
  • verified DataFrame.cast<T>(verify = true) / castTo(..., verify = true) behavior
• Explain that verify = true checks schema compatibility but does not perform data conversion.
• Mention convertTo as the right API when columns need to be converted to match a schema.
• Improve the site page with clearer examples and data tables, following the style of other API pages.
• Add or improve unit tests for the main overloads and edge cases.

Test scenarios as an option

Successful scenarios

• DataFrame<*>.cast<T>(verify = false) changes the formal schema type without validating data.
• DataFrame<*>.cast<T>(verify = true) succeeds when the dataframe matches the target schema.
• DataFrame<*>.castTo(schemaFrom = df, verify = true) reuses schema from another dataframe.
• DataFrame<*>.castTo(schemaFrom = ::factory, verify = true) reuses schema from a function returning a typed dataframe.
• DataRow<*>.cast<T>(verify = true) succeeds when the parent dataframe matches the schema.
• Column-level cast overloads preserve the same underlying column while changing the formal type.

Error scenarios

• cast<T>(verify = true) throws when a required column is missing.
• cast<T>(verify = true) throws when a column has an incompatible type.
• cast<T>(verify = true) throws or rejects incompatible nested column group structure.
• castTo(..., verify = true) follows the same validation behavior as cast<T>(verify = true).

Acceptance criteria

• Public cast / castTo APIs in cast.kt have concise KDocs.
• KDocs distinguish unchecked casts from verified schema checks.
• Site docs contain clearer examples and data tables for cast and castTo.
• Unit tests cover successful and failing schema verification cases.
• Tests cover at least one castTo overload.
• Docs point users to convertTo when actual data conversion is needed.