Remind people when GraphQL generated documentation is outdated

GraphQL documentation is generated manually using a rake task, when the schema changes
we need to regenerate the docs.

Proposal

To remind people to regenerate the docs we could use a job allowed to fail that runs the script rake gitlab:graphql:compile_docs when a commit is pushed and there are any differences in the docs we mark it as failed.

I am not sure if this is the best way to do it. Other solutions can be discussed here.