feat: add support for two column lists

[JakeSCahill]

Allow writers to define unordered lists that should span two columns to help with scanability:

- General purpose: General-purpose instances provide a balance of compute, memory, and networking resources, and they can be used for a variety of diverse workloads.
+
[.two-column]
** https://aws.amazon.com/ec2/instance-types/m5/[M5d^]
** https://aws.amazon.com/ec2/instance-types/m5/[M5ad^]
** https://aws.amazon.com/ec2/instance-types/m5/[M5dn^]
** https://aws.amazon.com/ec2/instance-types/m6g/[M6gd^]
** https://aws.amazon.com/ec2/instance-types/m7g/[M7gd^]

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