cli, docs: Add a docs command for doc generation

This took a lot longer than it looks to get right. It's not perfect, but
it now reliably generates documentation which we can put into gohugo.

lang/core/example/answer.go

@@ -42,8 +42,12 @@ const Answer = 42
 func init() {
 	simple.ModuleRegister(ModuleName, "answer", &simple.Scaffold{
 		T: types.NewType("func() int"),
-		F: func(context.Context, []types.Value) (types.Value, error) {
-			return &types.IntValue{V: Answer}, nil
-		},
+		F: TheAnswerToLifeTheUniverseAndEverything,
 	})
 }
+
+// TheAnswerToLifeTheUniverseAndEverything returns the Answer to Life, the
+// Universe and Everything.
+func TheAnswerToLifeTheUniverseAndEverything(context.Context, []types.Value) (types.Value, error) {
+	return &types.IntValue{V: Answer}, nil
+}