Add group SAML configuration example

What does this MR do?

We recently had a customer (ZD (internal use only) who was asking how to configure Group SAML with the GitLab Chart.

Due to how simple the configuration is in Omnibus https://docs.gitlab.com/ee/integration/saml.html#configuring-group-saml-on-a-self-managed-gitlab-instance, it wasn't clear if Group SAML needed to be added to a secret (it does).

Adding to the docs to save someone else confusion later.

Related issues

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course
• Follow the:
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  • Style Guide.
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If you are only adding documentation, do not add any of the following labels:

• ~"feature"
• ~"frontend"
• ~"backend"
• ~"bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

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