Add instructions for setting up Custom Domain TXT Verification ID before deploying Azure Container App

[tjementum]

It is now possible to get the Custom Domain TXT Verification ID before creating the Azure Container App Environment (see API here). That will make it possible to configure the TXT DNS Record before starting the first deployment of Infrastructure, potentially avoiding this ugly hack in the deployment script. It's also a requirement to create a CNAME record, and we will have to investigate if this can be done after deployment.

Please note that we will need to register the Microsoft.ApiManagement provider before calling this API.