adding moosedocs for extra id vpp, refs idaholab#19217

modules/reactor/doc/content/source/vectorpostprocessors/ExtraIDIntegralVectorPostprocessor.md

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+# ExtraIDIntegralVectorPostprocessor
+
+!syntax description /VectorPostprocessors/ExtraIDIntegralVectorPostprocessor
+
+
+## Overview
+
+The `ExtraIDIntegralVectorPostprocessor` object is a vector postprocessor to integrate input variables based on multiple extra IDs.
+First, it finds unique combinations of extra IDs, and then it computes separate integral values over input variables for elements with these unique combinations.
+For reactor applications, reactor component-wise values, such as pin-by-pin power distribution can be easily tallied using this object.
+
+The `ExtraIDIntegralVectorPostprocessor` object needs the following parameters:
+
+- [!param](/VectorPostprocessors/ExtraIDIntegralVectorPostprocessor/variable): variables that this VectorPostprocessor operates on.
+
+- [!param](/VectorPostprocessors/ExtraIDIntegralVectorPostprocessor/id_name): list of extra IDs by which to separate integrals
+
+## Example Syntax
+
+!listing modules/reactor/test/tests/vectorpostprocessors/extra_id_vpp.i block=VectorPostprocessors
+
+!syntax parameters /VectorPostprocessors/ExtraIDIntegralVectorPostprocessor
+
+!syntax inputs /VectorPostprocessors/ExtraIDIntegralVectorPostprocessor
+
+!syntax children /VectorPostprocessors/ExtraIDIntegralVectorPostprocessor