Added GroupBy transformation KDocs

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/add.kt

@@ -12,6 +12,7 @@ import org.jetbrains.kotlinx.dataframe.annotations.AccessApiOverload
 import org.jetbrains.kotlinx.dataframe.annotations.HasSchema
 import org.jetbrains.kotlinx.dataframe.annotations.Interpretable
 import org.jetbrains.kotlinx.dataframe.annotations.Refine
+import org.jetbrains.kotlinx.dataframe.api.GroupByDocs.Grammar
 import org.jetbrains.kotlinx.dataframe.columns.BaseColumn
 import org.jetbrains.kotlinx.dataframe.columns.ColumnAccessor
 import org.jetbrains.kotlinx.dataframe.columns.ColumnPath
@@ -429,6 +430,10 @@ public fun <T> DataFrame<T>.add(body: AddDsl<T>.() -> Unit): DataFrame<T> {
  * Returns a new [GroupBy] with the new column
  * appended to each group [DataFrame] to the original list of [DataFrame.columns].
  *
+ * Check out [`GroupBy grammar`][Grammar].
+ *
+ * For more information: {@include [DocumentationUrls.GroupByTransformation]}
+ *
  * ## Example
  *
  * ```kotlin
@@ -445,8 +450,6 @@ public fun <T> DataFrame<T>.add(body: AddDsl<T>.() -> Unit): DataFrame<T> {
  * }
  * ```
  *
- * For more information: {@include [DocumentationUrls.Add]}.
- *
  * @param name name for a new column.
  * If it is empty, a unique column name will be generated.
  * Otherwise, it should be unique for original group [DataFrame]s.

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/groupBy.kt

@@ -109,6 +109,8 @@ internal interface GroupByDocs {
      * {@include [Indent]}
      * `\[ `__`.`__[**`add`**][GroupBy.add]**`(`**`column: `[`DataColumn`][DataColumn]**`)  {  `**`rowExpression: `[`RowExpression`][RowExpression]**`  }  `**`]`
      *
+     * See [GroupBy Transformations][Transformation].
+     *
      * ### Reduce [GroupBy] into [DataFrame]
      *
      * {@include [Indent]}
@@ -138,6 +140,8 @@ internal interface GroupByDocs {
      * {@include [Indent]}
      * `| `__`.`__[**`values`**][ReducedGroupBy.values]**`  {  `**`valueColumns: `[`ColumnsSelector`][ColumnsSelector]**` }`**
      *
+     * See [GroupBy Reducing][Reducing].
+     *
      * ### Aggregate [GroupBy] into [DataFrame]
      *
      * {@include [Indent]}
@@ -164,6 +168,8 @@ internal interface GroupByDocs {
      * {@include [Indent]}
      * `| `__`.`__[<aggregation_statistic>][AggregationStatistics]
      *
+     *  See [GroupBy Aggregations][Aggregation].
+     *
      * ### Pivot [GroupBy] into [PivotGroupBy] and reduce / aggregate it
      *
      * {@include [Indent]}
@@ -240,11 +246,14 @@ internal interface GroupByDocs {
      * * [sortBy][GroupBy.sortBy] / [sortByDesc][GroupBy.sortByDesc] — sorts the **order of rows within each group**
      *   by one or more column values;
      * * [updateGroups][GroupBy.updateGroups] — transforms each group into a new one;
-     * * [filter][GroupBy.filter] — filters group rows by the given predicate (as usual [DataFrame.filter]);
+     * * [filter][GroupBy.filter] — filters out keys and corresponding groups that
+     *   do not satisfy the given key-group predicate;
      * * [add][GroupBy.add] — adds a new column to each group.
      *
      * Each method returns a new [GroupBy] with updated group order or modified group content.
      *
+     * Check out [`GroupBy grammar`][Grammar].
+     *
      * For more information: {@include [DocumentationUrls.GroupByTransformation]}
      */
     typealias Transformation = Nothing
@@ -545,6 +554,10 @@ public interface GroupBy<out T, out G> : Grouped<G> {
      * Creates a new [GroupBy] by transforming each group’s [DataFrame]
      * using the provided [transform] function.
      *
+     * Check out [`GroupBy grammar`][Grammar].
+     *
+     * For more information: {@include [DocumentationUrls.GroupByTransformation]}
+     *
      * __NOTE:__ This operation removes key-column status from each column in the group.
      * In other words, each column in the group is treated as a new column,
      * and not omitted when [`.values()`][Grouped.values] or other aggregations are called.
@@ -561,6 +574,10 @@ public interface GroupBy<out T, out G> : Grouped<G> {
      * The [predicate] is a [GroupedRowFilter], which behaves similarly to a [RowFilter] used in [DataFrame.filter],
      * but also provides access to the [group][GroupedDataRow.group] in the current row.
      *
+     * Check out [`GroupBy grammar`][Grammar].
+     *
+     * For more information: {@include [DocumentationUrls.GroupByTransformation]}
+     *
      * ### Example
      * ```kotlin
      * // Keep only key–group pairs where the "category" key equals "Engineer"