There's currently no ability to set a description on the SDL `schema` declaration

This is a nice place to introduce your overall schema, but we can't currently add it via Grats.

Start by locating Grats's handling of the SDL schema declaration and compare it with any existing support for descriptions on other declarations. Confirm how the schema description is represented in the generated GraphQL output, then add coverage showing that a description can be set and appears correctly.

Written by the indexing model from the issue text.