Add reference to z/OS on the main runner install page

Darren Eastmanrequested to merge
DarrenEastman-main-patch-46c8 into main

What does this MR do?

Add a reference to z/OS on the main runner install page

Why was this MR needed?

What are the relevant issue numbers?

https://gitlab.com/gitlab-com/ops-sub-department/zos-runner/phase-1-certification/-/issues/13

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