A lot of resources (like logos, images, wallpapers, slides) are also available, with sources, on a dedicated repository on Dolibarr GitHub. You can navigate into directories according to what you are looking for. All resources in this repository are provided under the CC-By-SA license.
So you can use, modify and redistribute these resources even for commercial purposes.
The Github dedicated to Dolibarr resources is at: https://github.com/Dolibarr/dolibarr-foundation
Our video resources (tutorials, demos) are available on our Youtube Channel
Some free online courses are available on the Dolibarr Academy Platform
If you are looking for documentation, take a look at this dedicated page