Update examples docs/examples to hook nexus generate on post install hook

[jasonkuhrt]

Users get no typegen typechecking after cloning a repo. All examples should use the post install npm hook to run nexus typegen. Also we should update the nexus workflow docs to mention this tip.

• https://github.com/prisma-labs/nexus/tree/develop/examples
• https://github.com/prisma-labs/nexus-prisma/tree/master/examples
• website docs

Depends on prisma-labs/graphql-framework-experiment#6