Add flowchart for home network setup in documentation

[MariaBego56]

Description

Added a flowchart diagram to illustrate a simple home network setup for WireGuard VPN. I replaced the existing Markdown ASCII diagram by a Mermaid one to make maintenance easier. Peer-to-site.

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• Fixes: Graphics: Replace ascii-art diagrams #118

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• [x ] I have read and followed the Ubuntu Server contributing guide.
• [ x] I have signed the Contributor License Agreement (CLA).
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Additional Notes (Optional)

I have added an extra internal device the user may have at home, I called it Y. This was to reinforce the impression that users may have multiple internal devices beyond those mentioned in the guide.

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[s-makin]

To simplify merging, I've applied these changes to #391 instead - I will now close this PR and we'll work off 391