Swap remaining Code Owners image to Mermaid

What does this MR do?

In !92818 (merged) I replaced the first of two images in https://docs.gitlab.com/ee/user/project/code_owners.html#groups-as-code-owners with a Mermaid graph. Let's do the second -- trickier -- one.

Before

After (in progress)

Observations:

• I took out Project B. I think it's extraneous here.
• conveying the decision points requires me to break standard formatting somewhat, because we're not interested in the no outcomes for the two options.
• would it be clearer to change the phrasing to Invite subgroup Y to Project A? and Add subgroup Y as Code Owners for project A? Length vs clarity, discuss!

graph LR
    A[Parent group X] -->|owns| B[Project A]
    A -->|contains| C[Subgroup Y]
    C -.->D{Invite to<br/>Project A?} -.->|Approvals can be<br/> required or optional| B
    C -.->E{Add as Code Owners<br/>to Project A?} -.->|Approvals can only<br/>be optional| B

Related issues

Related to Swap codeowner image out for Mermaid diagram (!92818 - merged)

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