Add documentation for Geometry3d

Warning, you must merge #287 before this pr.
This pr contains the documentation of:
* `ST_TriangleAspect`
* `ST_TriangleContouring`
* `ST_TriangleDirection`
* `ST_TriangleSlope`
* `ST_Force3D`

See #289
@ebocher I don't understand this result. I think it should be `0.0`.