Consider redrawing architecture diagram using Mermaid

@gctucker [said earlier](https://github.com/kernelci/kcidb/pull/238#discussion_r753338087):
> ASCII art is cool, but for this static website there's also Mermaid which is easier on the eyes.
>
> See this example:
> https://github.com/kernelci/kernelci-project/blame/main/kernelci.org/content/en/docs/architecture.md#L53
> and how it's rendered:
> https://kernelci.org/docs/architecture/

Consider using that to make the architecture diagram nicer in `doc/administrator_guide.md`. One drawback, is that it becomes unreadable in the raw Markdown file, kinda defeating its original idea, but perhaps we could live with that for the sake of a prettier website.