docs: add service principal auth docs for azure pipelines scaler

[m-amaresh]

Update the unreleased Azure Pipelines scaler documentation for the new Azure AD service principal authentication support.

This PR documents:

• service principal authentication with tenantId + clientId + clientSecret
• service principal authentication with tenantId + clientId + clientCertificate (+ optional clientCertificatePassword)
• continued support for personalAccessToken
• continued support for Azure Workload Identity

It also updates the authentication section to make the auth modes and validation rules explicit, and adds examples for:

• service principal authentication using client secret
• service principal authentication using client certificate
• Azure Workload Identity authentication

Related

Relates to kedacore/keda#7578

Notes

• updated unreleased docs under content/docs/2.20/
• tenantId and clientId are required for service principal authentication
• clientSecret and clientCertificate are mutually exclusive
• clientCertificatePassword is only valid when clientCertificate is provided

Checklist

• [*] Commits are signed with Developer Certificate of Origin (DCO)

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