Add optional GDK pixbuf plugins to the documentation

Until today I didn't know how easily one can extend the recognized file formats of Ristretto. There are very nice GdkPixbuf loader modules available:

These two are available via the official Ubuntu repositories and also have exactly this name in Synaptic:

• libavif-gdk-pixbuf (.avif support)
• heif-gdk-pixbuf (.heic and .heif support)

This one I compiled by myself but is also available via PPA:

• WebP GDK Pixbuf Loader library

So can we explain this here? We should explain the benefit and how to install the plugins for the main Linux distributions.