Add some diagrams to compas.geometry documentation

# 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.