Further restructuring,

docs/source/examples_simple_2D.rst

@@ -1,6 +1,6 @@
-****************************
-Introductory/basic 2D models
-****************************
+************************
+Introductory models (2D)
+************************
 
 This section provides some introductory example models in 2D that demonstrate basic features of gprMax. Each example comes with an input file which you can download and run.