Replace ASCII art with mermaid diagrams in WireGuard section

[s-makin]

Description

Unfortunately the original set of PR(s) that updated the ASCII art in the WireGuard section became quite complicated to merge due to a large number of conflicts with subsequent changes.

I have manually applied all of the changes proposed by @MariaBego56 onto a fresh/clean branch to avoid all of these conflicts, and it should be now good to merge.

The diagrams should now all be rendering correctly, and I have also fixed the issue that prevented RTD from building the 391 PR.

Thank you again, @MariaBego56 for creating these diagrams, and for all your patience as we worked through all the different build problems! Mermaid can be tricky, but we got there in the end.

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Related Issue

• Fixes: Graphics: Replace ascii-art diagrams #118
• Fixes: Ubuntu Server: replace ASCII-art diagrams open-documentation-academy#161
• Closes: Replace WireGuard VPN ASCII with mermaid diagrams #391

Checklist

• I have read and followed the Ubuntu Server contributing guide.
• My pull request is linked to an existing issue (if applicable).
• I have tested my changes, and they work as expected.

[MariaBego56]

Thank you @s-makin :-)