Document Feature Band support policy

We only release and support the latest available .NET SDK feature band at any given time. For example if SDK releases 6.0.100, 6.0.200, and 6.0.300, we will only release .NET Docker images for 6.0.300. This policy currently isn't documented anywhere but it should be explained under https://github.com/dotnet/dotnet-docker/blob/main/documentation/supported-tags.md#tag-lifecycle.