Add KDocs for `transpose` / `transposeTo`

[zaleslaw]

Add KDocs for transpose / transposeTo

The file core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/transpose.kt has two public, undocumented functions. Please add concise KDocs for both.

Scope

• DataRow<*>.transpose() — turns a single row into a two-column DataFrame<NameValuePair<*>>, with a name column (the original column names) and a value column (the row's values).
• DataRow<*>.transposeTo<T>() — the same, but converts every value to type T, producing a DataFrame<NameValuePair<T>>.

KDocs should explain

• What is produced: one output row per column of the source row, with name / value columns.
• The difference between the two: transpose() keeps values as-is (NameValuePair<*>); transposeTo<T>() converts each value to T.
• That the resulting schema is NameValuePair.

Acceptance criteria

• Both transpose() and transposeTo() have concise KDocs.
• The KDocs make the as-is vs converted-to-T distinction clear.