[FIX]: DPV logo broken in 404 page

[bact]

Specs

Other

Errors

Fix DPV logo at the top-left corner is broken if the path in the URL is more than one level:

• https://w3c.github.io/dpv/dpv - Logo OK
• https://w3c.github.io/dpv/dpv/ - Logo breaks

This is because the image uses relative path diagrams/logo.png,
have to fix that. Maybe use the absolute path.

[coolharsh55]

this should be fixed when dev is merged into master (Review and close)