build(aio): big move of docs related files (#14361)

All the docs related files (docs-app, doc-gen, content, etc)
are now to be found inside the `/aio` folder.

The related gulp tasks have been moved from the top level
gulp file to a new one inside the `/aio` folder.

The structure of the `/aio` folder now looks like:

```
/aio/
  build/         # gulp tasks
  content/       #MARKDOWN FILES for devguides, cheatsheet, etc
    devguides/
    cheatsheets/
  transforms/    #dgeni packages, templates, etc
  src/
    app/
    assets/
    content/    #HTML + JSON build artifacts produced by dgeni from /aio/content.
                #This dir is .gitignored-ed
  e2e/           #protractor tests for the doc viewer app
  node_modules/ #dependencies for both the doc viewer builds and the dgeni stuff
                #This dir is .gitignored-ed
  gulpfile.js   #Tasks for generating docs and building & deploying the doc viewer
```

Closes #14361

aio/transforms/angular.io-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);
+    }
+  };
+};