Add employee hierarchy example

[christycao2025]

This PR introduces the following updates:

• Employee Dummy Data:
  Added sample employee data for development and testing purposes. This data includes basic fields such as employee id, name, position, manager_id.

• Employee Hierarchy PySpark Script:
  Introduced a PySpark script that constructs the employee hierarchy using self-referencing joins on the dummy data.

[salesforce-cla]

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[salesforce-cla]

Thanks for the contribution! It looks like @christycao2025 is an internal user so signing the CLA is not required. However, we need to confirm this.