Don't install gem documentation
What does this MR do and why?
This speeds up the gem installation by skipping the documentation files which we don't need.
How to set up and validate locally
Numbered steps to set up and validate the change are strongly suggested.
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Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md. -
Check that setup still works: make setup