docs(charts-gitlab-runner): support to avoid mounting any key from the token K8s secret

What does this MR do?

Due to the need addressed in the gitlab-runner helm chart repository(issue: gitlab-runner#456 (closed), MR: gitlab-runner!399 (closed)), it will be necessary to update the documentation related to the proposed changes.

Related issues

These changes are to address issue #4714 (closed).

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