docs: what-if update with ValidationLevel

[matsest]

Is your feature request related to a problem? Please describe.

Can't seem to find any docs related to ValidationLevel options for What If deploys (see end of this thread https://github.com/Azure/arm-template-whatif/issues/135).

Describe the solution you'd like

Documentation for ValidationLevel for WhatIf deployments to be updated on

https://learn.microsoft.com/en-us/azure/azure-resource-manager/bicep/deploy-what-if?tabs=azure-powershell%2CCLI

https://github.com/MicrosoftDocs/azure-docs/blob/main/articles/azure-resource-manager/bicep/deploy-what-if.md

Should also clarify what permissions is needed when using providerNoRbac (my own findings indicate that as of now this requires a custom role if you want to do a what-if without write permissions - correct?)