Add some diagrams to compas.geometry documentation

[yck011522]

Feature Request

As a [user], I want [to understand how geometry primitives are parametrized] so that [I don't have to trail and error].

Details

Is your feature request related to a problem? Please describe.
I was trying to understand if compas.geometry.Cylinder is defined with a circle at the middle of the cylinder (extruding to both sides of the circle) or at the base of the cylinder.

Describe the solution you'd like
I would like a simple diagram so I can understand it immediately.

Describe alternatives you've considered
Trail and error / Looking into the implementation is time consumming.

Additional context
For illustrative purposes of what I mean by diagram, I found the following image on Google.

[beverlylytle]

This is perhaps related to #779.