Minimal doc changes for 3.0

[einarf]

Minimal tasks for 3.0 release. Some of these can be done incrementally

• [ ] Do a pass on front page
• [ ] Fill out the about section
• [ ] Fill out the contributing section
• [ ] Fill out the community section
• [ ] Resolve differences between arcade book and the skill tree
• [ ] Organize the api references. These needs to be put in categories one level down
• [ ] Organize the manual

Still, moving things can create future issues when users move between doc versions. We should try to establish structures as soon as possible.