user documentation: added powertrain structure

Documentation/User Manual/3-simulation-models/powertrain.md

+## Powertrain and Components Structure
+The powertrain in Vecto V3 consists of the following components which are connected in this order:
+
+![](pics/powertrain.svg)
+
+The engine tries to supply the requested power demand (including all power losses happening in the powertrain and auxiliaries).
+If the engine can't supply the given power demand, the driver component limits it (e.g. by accelerating slower than requested by the driving cycle).

Documentation/User Manual/3-simulation-models/simulation-models.md

@@ -2,6 +2,7 @@
 
 In this chapter the used component models for the simulation are described.
 
+* [Powertrain and Components Structure](#powertrain-and-components-structure)
 * [Driver: Acceleration Limiting](#acceleration-limiting)
 * [Driver: Look-Ahead Coasting](#look-ahead-coasting)
 * [ADAS: Overspeed / Eco-Roll](#overspeed-eco-roll)

Documentation/User Manual/files.txt

@@ -20,6 +20,7 @@
 2-calculation-modes/engine-only.md
 2-calculation-modes/batch.md
 3-simulation-models/simulation-models.md
+3-simulation-models/powertrain.md
 3-simulation-models/Driver_AccLimit.md
 3-simulation-models/Driver_LAC.md
 3-simulation-models/ADAS_EcoRoll.md