Draft: Update Cells: CI Runners architectural blueprint

What does this MR do and why?

As described in this issue the Tenant scale group needs input from the CI section as to how CI and runners should function in a Cells architecture. The goal is to use this draft architectural blueprint to (a) get a shared understanding between the CI section and the Tenant scale group and (b) to ensure that the critical subject matter experts in the ci_section participate in the architectural proposal discussion so that out of the gate we are making the right decisions.

#434981 (closed)

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