docs: revamp README and usage guide

[syall]

Currently the smithy-go README is extremely minimal and has gaps in terms of DX.

This issue captures updating the README portion, e.g. #468

[michael-cico]

This is really needed, also in terms of user docs.

Are there any cookbook-style examples on how to set up the plugin for local usage of the plugin? I haven't found much. I was able to build this repo locally, but when I try the example under codegen/smithy-go-codegen-test I get

SUCCESS: Validated 209 shapes

Validated model, now starting projections...

Unable to find a plugin for `go-codegen` in the `source` projection. Is this the correct spelling? Are you missing a dependency? Is your classpath configured correctly?