Add note for SaaS clients using PlantUML integration

What does this MR do and why?

The MR adds a note for SaaS customers that PlantUML integration is enabled by default.

One client in the ticket was asking about the feature and was referring to the GitLab documentation. Our documentation has instructions for enabling the integration, but does not mention neither SaaS or Self-Managed customers, which creates confusion.

How to set up and validate locally

Add below code block (in SaaS account) to md file and PlantUML diagram is rendered properly

```plantuml
Bob -> Alice : hello
Alice -> Bob : hi
```

MR acceptance checklist

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