Clarify section on becoming a Runner team maintainer

Why is this change being made?

This MR is a follow-up to !76213 (merged) to avoid confusion between the Onboarding and the Becoming a maintainer for one of our projects sections.

I moved the Becoming a maintainer for one of our projects section close to the Onboarding section since it seems they seem to be logically connected, and make reading the team process flow more easily. I think the text in that section could still be improved to clarify the difference between having maintainer access and achieving maintainer status. Suggestions welcome! cc @gitlab-com/runner-group

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
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