Add KDoc for `Implode`

Add KDoc for implode

Add KDocs for public non-deprecated implode overloads in:
core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/implode.kt

Issue: https://github.com/Kotlin/dataframe/issues/1385
Website page: https://kotlin.github.io/dataframe/implode.html
Docs source: docs/StardustDocs/topics/implode.md

The KDoc should be aligned with the existing website page: implode returns a DataFrame where values in selected columns are merged into lists grouped by the remaining columns.

Cover overloads:

• DataFrame<T>.implode(dropNA: Boolean = false): DataRow<T>
• DataFrame<T>.implode(dropNA: Boolean = false, columns: ColumnsSelector<T, C>): DataFrame<T>
• DataFrame<T>.implode(vararg columns: String, dropNA: Boolean = false): DataFrame<T>

Do not spend effort on deprecated access API overloads except keeping them consistent if needed.

Mention key behavior:

• selected columns are imploded, other columns are used as grouping keys;
• dropNA = true removes NA values from merged lists;
• value columns change type from T to List<T>;
• DataRow / ColumnGroup values are converted into dataframe/frame-column representation, as described on the website;
• implode is the reverse operation of explode.

Add a documentation URL snippet for implode to DocumentationUrls if missing, similar to Explode.

Use explode.kt KDocs as the closest style/reference.