[doc] Update Codegen doc to use Server Preset

Hello
I'm from The Guild and I want to propose updating the Generating TS types to use the Server Preset instead of the current setup.

This is now our recommended way to write server code, and I believe it gives users much better DevX:

• has automatic type wireup for better type-safety
• results in less runtime errors
• easier typeDefs and resolvers setup

I'm thinking to keep the similar flow to the current doc and only update relevant bits to keep it short. Happy to chat more!