Add onboarding doc for typespec-go

[jhendrixMSFT]

Be sure to include info about post-codegen customization.

[lirenhe]

@jhendrixMSFT, we already had some document in https://azure.github.io/typespec-azure/docs/howtos/generate-client-libraries/00howtogen/

Could you provide more details about the info needed in this doc and is this for data plane or mgmt. plane?

[jhendrixMSFT]

Thanks, I forgot that we had this doc.

I took a look, and I think we need to add a section about adding post-codegen customization.