Display L1 constructs in a separate section

[gshpychka]

Currently, when looking at a submodule of CDK, L1 construct classes are mixed with L2. I'd like to keep the behavior of the API reference, where they're shown in a separate section.

API reference:

Construct hub:

(I have a dark theme extension, disregard)

[Chriscbr]

I think it's still a good idea! I've definitely found this distinction helpful while browsing the AWS CDK API reference 👍