Consider excluding `azure_` prefix'd symbols in doc generation and update our pattern in C++ to use `azure_` instead of `az_`

[ahsonkhan]

We currently exclude az_ and AZ_ We should consider excluding azure_ and AZURE_ as well.

https://github.com/Azure/azure-sdk-for-cpp/blob/61f5ce00c2908d9017b2ad3e7891ced99fe67de0/cmake-modules/AzureDoxygen.cmake#L44-L47

From conversations with @antkmsft, he brought up an interesting point that we use the az_ prefix in the Embedded C SDK, so we may want to use a different prefix in the C++ SDK to avoid name collision if a customer is using both SDKs (i.e. azure_). We may also want to consider renaming the existing non-public use of the az_/AZ_ prefix.

This will let us skip things like AZURE_ASSERT which isn't meant for public use anyway. https://azuresdkdocs.blob.core.windows.net/$web/cpp/azure-core/1.11.2/azure__assert_8hpp.html#a23ae14b8c5b65e95fa2c61417450d840

That said, we have to be careful to check if we have any valid uses of azure_ in our public surface area.

cc @LarryOsterman, @RickWinter