[9.0] Add dashboard file config directory docs

[JamesNK]

Dashboard has a new config option. See https://github.com/dotnet/aspire/issues/4526

Note: The name changed in https://github.com/dotnet/aspire/pull/5777

Should be documented here: https://learn.microsoft.com/en-us/dotnet/aspire/fundamentals/dashboard/configuration?tabs=bash#common-configuration