Update codegen documentation to show smithy-trait-codegen

[kubukoz]

e.g. in https://smithy.io/2.0/guides/building-codegen/configuring-the-generator.html#converting-json-configuration-to-java

there's no mention of smithy-trait-codegen, and I think it's a pretty huge time-saver when it comes to writing these POJOs.

[kubukoz]

hmm I may've reacted too quickly by making this, in practice these things aren't "traits" per se. However, there's still Node parsing involved, so the codegen could make that easier.

[hpmellema]

Smithy trait codegen is still in an unstable state. We plan to add additional usage documentation to smithy.io as we stabilize the generator.

That said, as I see it, trait codegen doesn't really apply to settings nodes in Directed codegen. These settings nodes are not typically modeled in the IDL so wont be generatable. Additionally, trait codegen is specific to trait shapes and not really intended for generic Node parsing which seems to be what you are getting at.

[kubukoz]

Fair enough!