workflow/docs.yml: pin setup-ruby action

[Moisan]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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Pin the version of the setup-ruby action to full length commit SHA as described in the security hardening for GitHub Actions guide. I believe dependabot settings currently covers updating the setup-ruby commit SHA.

[MikeMcQuaid]

Thanks again @Moisan and for suggestions @Bo98!