Add more example GraphQL queries to the docs

[sqs]

As a user who is not familiar with GraphQL or the Sourcegraph GraphQL API, I want to see more example queries to perform common tasks in the Sourcegraph GraphQL API documentation examples page. Specifically, I would like to see an example of how to query for all discussion threads on a given file in a repository.

Requested by @P3GLEG

[stale]

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