A lot of resources (like logos, images, wallpaper, slides) are also available, with sources, on a dedicated repository of Dolibarr GitHub. You can navigated into directories according to what you are looking for. All resources on this repository are provided under licence CC-By-SA.
So you can use, modify and redistribute them even for a commercial usage..
To go onto the Github dedicated to Dolibarr resources: https://github.com/Dolibarr/dolibarr-foundation
Our videos resources are available into our Youtube Channel
Some free online courses are available into the Dolibarr Academy platform
If you are looking for documentation, take a look at this dedicated page