improve user-facing documentation

The Go Report Card for graphql-go/graphql shows a large number of exported symbols [without godoc comments](https://goreportcard.com/report/github.com/graphql-go/graphql#golint). Providing docs would certainly help user wanting to use the package.

Some of the smaller examples could also be moved into example tests given that pkg.go.dev renders them as main packages in the documentation. This would also ease uptake. If this is a desirable change, I can send a PR for this.