Update cells blueprint to reference groups rather than namespaces

What does this MR do and why?

Define how groups and namespaces differ within the context of organizations and cells.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes #404333 (closed)