[Feature Request] Include implementations of interfaces in documentation

**Is your feature request related to a problem? Please describe.**
Many of the schemas that I work with have interfaces that are implemented by several types. However, those types may not appear anywhere else in the schema documentation because they are not explicitly part of any operation.

For example, imagine you have a `Shape` interface, and `Circle` and `Square` types. But the `Query` object has a single method: `shapes` that returns the `Shape` interface. This means that `Circle` and `Square` are not available in the documentation.

**Describe the solution you'd like**
In the example above, the GraphiQL app would an `Implementations` section on the page for the `Shape` interface documentation. This is helpful because it gives the developer a way to determine what Shapes are available to get more detail from.

**Describe alternatives you've considered**
Another option is to have a list of all types somewhere in the documentation, but that makes it difficult to understand what types implement a specific interface.

Start by locating the GraphiQL app's interface documentation view and the schema data used to render it. Trace how interface types and their implementing types are exposed, then verify that an Implementations section lists those types on the interface documentation page.

Written by the indexing model from the issue text.