hugo: explain physics of roughness models

I added a very basic documentation of the new API to ref/sample/roughness. Some more work is needed to explain the physical meaning of models and parameters.

• Explain parameters of LinearGrowthModel
• Explain why we need an erf or tanh transient (see also !1129 (merged))
• Explain cross-correlation models

changed milestone to %v22

added docs release-critical labels

removed release-critical label

assigned to @m.svechnikov

mentioned in merge request !2957 (merged)

mentioned in commit 9ba98f44

closed with merge request !2957 (merged)