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/processors/createOverviewDump.js

@@ -0,0 +1,24 @@
+var _ = require('lodash');
+
+module.exports = function createOverviewDump() {
+
+  return {
+    $runAfter: ['processing-docs'],
+    $runBefore: ['docs-processed'],
+    $process: function(docs) {
+      var overviewDoc = {
+        id: 'overview-dump',
+        aliases: ['overview-dump'],
+        path: 'overview-dump',
+        outputPath: 'overview-dump.html',
+        modules: []
+      };
+      _.forEach(docs, function(doc) {
+        if ( doc.docType === 'module' ) {
+          overviewDoc.modules.push(doc);
+        }
+      });
+      docs.push(overviewDoc);
+    }
+  };
+};