Create unpivot gem docs

docs/Spark/gems/transform/unpivot.md

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+---
+title: Unpivot
+id: unpivot
+description: Use the Unpivot Gem to transform your data from a wide format to a long format.
+tags:
+  - gems
+  - unpivot
+  - wideformat
+  - longformat
+---
+
+Use the Unpivot Gem to transform your data from a wide format to a long format.
+
+## Parameters
+
+| Parameter                       | Description                                                                                                                  |
+| ------------------------------- | ---------------------------------------------------------------------------------------------------------------------------- |
+| Column(s) to use as identifiers | The column(s) that will identify to which group or entity the observation corresponds to.                                    |
+| Columns to unpivot              | The columns (wide format) that you would like to transform into a single column (long format).                               |
+| Variable column name            | The name of the column that contains the names of the unpivoted columns. This helps describe the values in the value column. |
+| Value column name               | The name of the column that will contain the values from the unpivoted columns.                                              |
+
+## Example
+
+Transforming your data into a long format can be beneficial when creating visualizations, comparing variables, handling dynamic data, and more.
+
+Let's think about a time series example. If you have product sales data in a wide format, you may want to transform it into a long format before modeling the time series and analyzing the seasonal patterns in sales.
+
+The image below shows sample input and output tables for this scenario.
+
+![Wide and long formats of time series data](./img/unpivot-time-series.png)
+
+This table describes how this transformation was achieved:
+
+| Parameter                       | Input                                                                                             |
+| ------------------------------- | ------------------------------------------------------------------------------------------------- |
+| Column(s) to use as identifiers | The _Product_ column is the identifier because it defines which product the sales correspond to.  |
+| Columns to unpivot              | All of the quarterly sales columns will be unpivoted.                                             |
+| Variable column name            | The variable column is named _Quarter_ because it identifies the sales period.                    |
+| Value column name               | The value column is named _UnitsSold_ because it contains information about number of units sold. |