Updates impact.rst with landing content

Author: spjuhel

doc/user-guide/impact.rst

@@ -2,13 +2,21 @@
 Impact Tutorials
 ================
 
+These tutorials show how to compute impacts with CLIMADA, and all related aspects such 
+as impact functions, adaptation measures and discount rates and cost-benefit analysis.
+
+The first tutorial presents an end-to-end impact calculation, 
+and subsequent ones present each aspect in more details.
+
+Additionally you can find a guide on how to populate impact data from EM-DAT database.
+
 .. toctree::
     :maxdepth: 1
 
     Impact Calculation <climada_engine_Impact>
     climada_entity_ImpactFuncSet
     climada_entity_MeasureSet
     Discount Rates <climada_entity_DiscRates>
-    climada_engine_impact_data
+    Using EM-DAT data <climada_engine_impact_data>
     Cost Benefit Calculation <climada_engine_CostBenefit>
     Probabilistic Yearly Impacts <climada_util_yearsets>