Fix vignette title and improve vignette & logo detection

[Bisaloo]

This PR was prompted by some misc issues on our new r-universe:

• https://epiverse-trace.r-universe.dev/ui#packages: @thibautjombart's face appears instead of the package logo, while this should only happen when the package doesn't have a logo (or an opengraph image)
• https://epiverse-trace.r-universe.dev/ui#articles: the vignette title is not correct

[thibautjombart]

Thanks for the PR! I love that it is concise and to the point. :)