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/cheatsheet-package/processors/createCheatsheetDoc.js

@@ -0,0 +1,48 @@
+var _ = require('lodash');
+
+module.exports = function createCheatsheetDoc(
+    createDocMessage, renderMarkdown, versionInfo, targetEnvironments) {
+  return {
+    $runAfter: ['processing-docs'],
+    $runBefore: ['docs-processed'],
+    $process: function(docs) {
+
+      var currentEnvironment = targetEnvironments.isActive('ts') && 'TypeScript' ||
+          targetEnvironments.isActive('js') && 'JavaScript' ||
+          targetEnvironments.isActive('dart') && 'Dart';
+
+      var cheatsheetDoc = {
+        id: 'cheatsheet',
+        aliases: ['cheatsheet'],
+        docType: 'cheatsheet-data',
+        sections: [],
+        version: versionInfo,
+        currentEnvironment: currentEnvironment
+      };
+
+      docs = docs.filter(function(doc) {
+        if (doc.docType === 'cheatsheet-section') {
+          var section = _.pick(doc, ['name', 'description', 'items', 'index']);
+
+          // Let's make sure that the descriptions are rendered as markdown
+          section.description = renderMarkdown(section.description);
+          section.items.forEach(function(item) {
+            item.description = renderMarkdown(item.description);
+          });
+
+
+          cheatsheetDoc.sections.push(section);
+          return false;
+        }
+        return true;
+      });
+
+      // Sort the sections by their index
+      cheatsheetDoc.sections.sort(function(a, b) { return a.index - b.index; });
+
+      docs.push(cheatsheetDoc);
+
+      return docs;
+    }
+  };
+};