Adds runnable samples under samples/PivotTable demonstrating the pivot table
feature, discovered automatically by the sample browser:
- 01_Create_PivotTable: build a basic pivot (row/column/value fields) from a
source range and save it as Xlsx.
- 02_PivotTable_Page_Filter: add a page (report filter) field.
- 03_PivotTable_Grouping: group a numeric field into ranges and a date field
by quarter.
- 04_Read_PivotTable: write a pivot, load it back, and inspect the pivot table
object model (source, row/column/page fields, value aggregation).
Directory is named PivotTable to match the PascalCase convention used by the
other sample categories (Table, Chart, ...).
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