Add a doc page about user expectations

Add a page describing what users need to know in order to perform different tasks using compas_fab. This can go on the first introduction page or a deliciated page of its own.

**Level 0 (Complete Layman / No Code)**
Visualization

**Level 1 (Typical users)**
Define Robot Cell (Robot, Collision Environment, Tools, Workpieces)
Plan Motion
Plan Process

**Level 2 (Advanced User)**
Modify Robot Kinematics and Semantics (e.g. Planning Groups)
Custom Robot (e.g. multiple robots in cell)

**Level 3 (Research level)**
Extension with custom Motions and Planning Algorithms
Extension with custom Planning Backends
Configure planning parameters for individual planning backends