Add the yaml file for PowerShell Updatable Help (CabGen) CI

[jeffreyxrh]

We recently added cabgen-bootstrap.yml (a pipeline source file) to PowerShell Reference template so that repos on Docs Portal can include it after initialization. This repo is initialized before, so we manually add it now, in case you want to onboard PowerShell Updatable Help (CabGen) CI in the future.
This PR is to main branch, but it should be merged to live branch before setting up the CI.
For details, see this work item at Azure DevOps: https://dev.azure.com/ceapex/Engineering/_workitems/edit/608278

[opbld32]

Docs Build status updates of commit 1dafe34:

✅ Validation status: passed

File	Status	Preview URL	Details
cabgen-bootstrap.yml	✅Succeeded

For more details, please refer to the build report.

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