Better cross-refs between CODEOWNERS and rules

What does this MR do?

Susan had questions about the Code Owners (https://docs.gitlab.com/ee/user/project/code_owners.html) page:

• WHY would I not want to use Code Owners for approvals?
• In what circumstances would I want to use it? I’m trying to understand the why.
• Does an either/or exist between Code Owners and approvals?
• Can we note that you're limited to one CODEOWNERS file?

Also, better cross-referencing between the two pages.

I'd also like Kai to double-check my facts.

Related issues

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