Add back commonmarker

What does this MR do and why?

!192279 (merged) removed the commonmarker gem. The GitHub import code relies on it, but the graphql-docs gem includes it. However, that gem is only enabled in development and test, so we were having failures with import. See !192279 (comment 2544414019).

Adding gem back until #545120 is completed.

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How to set up and validate locally

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