Swap codeowner image out for Mermaid diagram

What does this MR do?

Attempts to replace the first image in https://docs.gitlab.com/ee/user/project/code_owners.html#groups-as-code-owners with a Mermaid graph.

Before

After

We got this right on the second try!

graph TD
    A[Parent group X] -->|owns| B[Project A]
    A -->|contains| C[Subgroup Y]
    C -->|owns| D[Project B]
    A-. inherits ownership .-> D

Related issues

Related to Add Note for group membership (!92190 - merged) which forced me to stare at the image, grit my teeth, and say "I'm doing something about it this time"

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