Update Azure Integrations docs to recommend AsExisting over AddConnectionString

[eerhardt]

Describe the issue or suggestion

In each of the Azure Integration docs there is a section for Connect to an existing Azure XX account. These sections are saying that if you want to connect to an existing Azure service, use the builder.AddConnectionString("name") method. This approach doesn't work as well as using .AsExisting because:

1. The connection string isn't typed, you don't know if it is for a KeyVault, Storage, etc.
2. Auth to the existing service isn't established. Each app gets a new Managed Identity created, and using a connection string doesn't add roles to this managed identity. But doing .AsExisting will add the necessary role assignments.

We should update these sections to recommend using AsExisting, like we have in the Azure Overview doc.

See also dotnet/aspire#9284

[IEvangelist]

Is the idea to replace the existing "Connect to an existing Azure XX account" via connection string, or just add and prioritize the AsExisting approach above the fold?

[eerhardt]

IMO I would remove the AddConnectionString content from each of the individual Integrations and replace it with the AsExisting approach. Then at the end of that section, a link back to "For more info see https://learn.microsoft.com/en-us/dotnet/aspire/azure/integrations-overview?tabs=dotnet-cli#use-existing-azure-resources" which has general info on AddConnectionString.

[IEvangelist]

Can this happen post build, or is it needed for 9.3?

[eerhardt]

Not required for 9.3.