Update documentation for adjust schema

[koperagen]

https://kotlin.github.io/dataframe/adjustschema.html
Suggestions:
Add information about castTo(schemaFrom: Function<DataFrame>) - kdoc provides an example,
and castTo(schemaFrom: DataFrame) - there's info in https://kotlin.github.io/dataframe/cast.html

This argument is used to generate [extension properties](https://kotlin.github.io/dataframe/extensionpropertiesapi.html) for typed data access. 

->

This arguments is used to resolve generated extension properties for typed data access.

[zaleslaw]

Update Documentation for adjustSchema

The adjustSchema documentation is currently too high-level and misses two important castTo(...) use cases that are already supported by the API.

Current page: https://kotlin.github.io/dataframe/adjustschema.html
Related page: https://kotlin.github.io/dataframe/cast.html

Problem

The page explains that DataFrame<T> uses T as a compile-time schema marker, but it only mentions cast and convertTo. It does not explain how users can reuse an existing schema marker with castTo(...).

It also says:

This argument is used to generate extension properties for typed data access.

This is not precise. The type argument itself does not generate extension properties. It is used to resolve generated extension properties for typed data access.

Suggested Changes

Update the wording to:

This argument is used to resolve generated extension properties for typed data access.

Add a short section about castTo(schemaFrom: DataFrame<T>):

val sample = DataFrame.readJson("sample.json")

for (file in files) {
    val df = DataFrame.readJson(file).castTo(sample)
    val count = df.count { perf > 10.0 }
    println("$file: $count")
}

This is useful when another DataFrame already has the expected schema marker and a newly loaded DataFrame should be treated as having the same schema.

Also add a section about castTo(schemaFrom: Function<DataFrame<T>>):

private fun convert(data: List<String>) = data.map { it.split(":") }.toDataFrame {
    "part1" from { it[0] }
    "part2" from { it[1].toInt() }
    "part3" from { it[2] }
}

fun deserializeAndUse(file: File) {
    val df = DataFrame.readJson(file).castTo(schemaFrom = ::convert)
    df.part1.print()
}

This is useful when the schema marker is inferred from a function returning DataFrame<T>, especially when the schema type is generated or otherwise inconvenient to reference directly as a type argument.

Acceptance Criteria

• adjustSchema.md explains cast, castTo, and convertTo at a high level.
• castTo(schemaFrom: DataFrame<T>) is mentioned with a link to cast.md.
• castTo(schemaFrom: Function<DataFrame<T>>) is documented with a short example.
• The wording about extension properties is corrected from “generate” to “resolve”.
• Examples are validated through the docs sample mechanism where possible.