SP with resource group level scope example

[billpratt]

What this PR does / why we need it:

Added example on how to create a service principal with a resource group level scope

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

If applicable:

• [ x ] documentation
• unit tests
• tested backward compatibility (ie. deploy with previous version, upgrade with this branch)

Release note:

[acs-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[billpratt]

/assign @brendandburns

[CecileRobertMichon]

lgtm

[CecileRobertMichon]

Thanks @billpratt!