Add documentation for Custom domain name journey

[abbottdev]

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

I'm trying to work out how to implement a custom domain when publishing to container apps - the documentation isn't really there but I can see there's been a bit of work to try and get something backported for 9.0 (using the PublishAsContainerApp / ConfigureCustomDomain methods.

I'm not sure if it's a gap, or whether I've not got the full story around the documentation but the journey for publishing an aspire app in a CI/CD pipeline wouldn't allow the ability to call azd provision without specifying custom publish parameters. It feels like if you want to use CI/CD you would be better off using the feature flag from azd: azd config set alpha.aca.persistDomains on (Azure/azure-dev#3955)

In any case, could we document the current recommended approach for custom domains somewhere? Potentially considering CI/CD scenarios, as well as the roadmap for this area generally so we can make some good design decisions upfront 😄

Previous issues (all closed, otherwise I'd have added to them!):
dotnet/aspire#6271
dotnet/aspire#1498
microsoft/azure-container-apps#796

TIA!

Describe the solution you'd like

Documentation outlining how to use and persist custom domains in a CI/CD scenario.

Additional context

No response