chore(doc-gen): put typescript stuff into its own package

This means that we can now run just the d.ts file generation by running:

```bash
gulp docs/typings
```

In addition the type definition generation was messing with the other docs tasks
so separating it also fixes problems there.

docs/docs-package/readers/ngdoc.js

@@ -0,0 +1,32 @@
+var path = require('canonical-path');
+
+/**
+ * @dgService ngdocFileReader
+ * @description
+ * This file reader will pull the contents from a text file (by default .ngdoc)
+ *
+ * The doc will initially have the form:
+ * ```
+ * {
+ *   content: 'the content of the file',
+ *   startingLine: 1
+ * }
+ * ```
+ */
+module.exports = function ngdocFileReader() {
+  var reader = {
+    name: 'ngdocFileReader',
+    defaultPattern: /\.md$/,
+    getDocs: function(fileInfo) {
+
+      // We return a single element array because ngdoc files only contain one document
+      return [{
+        docType: 'guide',
+        content: fileInfo.content,
+        startingLine: 1
+      }];
+    }
+  };
+
+  return reader;
+};