Clean up logos

[peterdesmet]

• [x] There are currently 5 logo versions in https://github.com/b-cubed-eu/documentation/tree/main/images, this should likely be cleaned up
• [x] Rename final logo to b3-logo.png
• [x] It's probably better to move the logo to /assets/images/
• [x] And update the link in the configuration to an absolute link /assets/theme/images/b3-logo.png https://github.com/b-cubed-eu/documentation/blob/ac38ff90f8564da79c21d8d897062d5249fa449f/_config.yml#L8