docs: Initiate building documentation with mkdocs

Also lay out the structure of the documentation and supply a decent
  introduction. Also (unsuccessfully) attempt to get the first example of
  a Stratego transformation from the manual implemented and documented.

syntax/Spoofax-Propositional-Language.sdf3

@@ -1,17 +1,33 @@
 module Spoofax-Propositional-Language
 
+/** md:
+Title: Concrete Syntax
+
+## Concrete Syntax
+
+The current Spoofax/Eclipse IDE is oriented around individual language
+projects. Therefore in order to work with ASTs of the SPL
+[abstract syntax](../README.md#abstract-syntax) that we just saw, it's easiest to
+define a concrete syntax for SPL, with the following SDF3 definition:
+
+*[SPL]: Spoofax Propositional Language
+
+```sdf3
+**/
+
 imports
   
   Common
 
-context-free start-symbols
-  
-  Prop
-
 context-free sorts
 
   Prop String
 
+context-free start-symbols
+
+  Prop
+
+/** md (include the following code in the documentation) */
 context-free syntax
   
   Prop.True  = <1>
@@ -32,4 +48,27 @@ context-free priorities
   > Prop.And
   > Prop.Or
   > Prop.Impl
-  > Prop.Eq
+  > Prop.Eq
+/* [stop to avoid the open-comment in the extracted markdown] **/
+
+/** md [conclusion of the documentation]
+```
+
+Thus, the first two example AST expressions
+
+`A: {! examples/sec4.1_A.aterm !}` and
+`B: {! examples/sec4.1_B.aterm !}`
+
+from {! ../docrefs/sec4.1.md !} can be generated by the following much more
+compact SPL expressions:
+
+`A: {! examples/sec4.1_A.spl !}` and
+`B: {! examples/sec4.1_B.spl !}`. (If you are following along in the repository,
+you can visit the file `syntax/examples/sec4.1_A.spl` (or ...`_B.spl`) and from the
+Spoofax menu select "Syntax > Show parsed AST" to see the AST generation
+in action.)
+
+This concrete syntax actually makes it easier to construct the example
+expressions used throughout the tutorial manual as examples for Stratego
+processing.
+**/

syntax/examples/sec4.1_A.spl

@@ -0,0 +1 @@
+(1 -> 0) & 0

syntax/examples/sec4.1_B.spl

@@ -0,0 +1 @@
+p & 0

syntax/examples/sec4.1_test1.spl

@@ -0,0 +1 @@
+(1 -> 0 & 1) & 1

syntax/examples/sec4.1_test2.spl

@@ -0,0 +1 @@
+(1 -> p & q) & p