Add a doc page about user expectations

[yck011522]

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