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.

editor/Main.esv

@@ -4,6 +4,7 @@ imports
 
   Syntax
   Analysis
+  Manual
 
 language
 

editor/Manual.esv

@@ -0,0 +1,7 @@
+module Manual
+
+menus
+
+  menu: "Manual" (openeditor)
+
+   action: "prop-eval" = do-eval

editor/Syntax.esv

@@ -20,6 +20,7 @@ menus
     
     action: "Format"          = editor-format (source)
     action: "Show parsed AST" = debug-show-aterm (source)
+    action: "Test action"     = debug-show-aterm
 
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