Improve documentation

[ufechner7]

• the term CAD frame is used, but not defined
• the term world frame is used, but not defined
• the wing is described as fixed, but it is flexible
• the term object is used, which should be replaced with struct
• the term elevation in the context of transform is not defined
• it is not clear where the physical model appears in the SystemStructure

Reference frames are already explained in KiteUtils and in VortexStepMethod. We should not duplicate this effort, but link it and make it easy to find from the documentation of KiteModels.jl.

Not so clear to me is, if this package should have its own Nomenclature page like VortexStepMethod or not.