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/examples-package/processors/collect-examples.js

@@ -0,0 +1,67 @@
+const {mapObject} = require('../utils');
+
+module.exports = function collectExamples(exampleMap, regionParser, log, createDocMessage) {
+  return {
+    $runAfter: ['files-read'],
+    $runBefore: ['parsing-tags'],
+    $validate: {exampleFolders: {presence: true}},
+    $process: function(docs) {
+      const exampleFolders = this.exampleFolders;
+      const regionDocs = [];
+      docs = docs.filter((doc) => {
+        if (doc.docType === 'example-file') {
+          try {
+            // find the first matching folder
+            exampleFolders.some((folder) => {
+              if (doc.fileInfo.relativePath.indexOf(folder) === 0) {
+                const relativePath =
+                    doc.fileInfo.relativePath.substr(folder.length).replace(/^\//, '');
+                exampleMap[folder] = exampleMap[folder] || {};
+                exampleMap[folder][relativePath] = doc;
+
+                const parsedRegions = regionParser(doc.content, doc.fileInfo.extension);
+
+                log.debug(
+                    'found example file', folder, relativePath, Object.keys(parsedRegions.regions));
+
+                doc.renderedContent = parsedRegions.contents;
+
+                // Map each region into a doc that can be put through the rendering pipeline
+                doc.regions = mapObject(parsedRegions.regions, (regionName, regionContents) => {
+                  const regionDoc =
+                      createRegionDoc(folder, relativePath, regionName, regionContents);
+                  regionDocs.push(regionDoc);
+                  return regionDoc;
+                });
+
+                return true;
+              }
+            });
+
+            return false;
+
+          } catch (e) {
+            throw new Error(createDocMessage(e.message, doc, e));
+          }
+        } else {
+          return true;
+        }
+      });
+
+      return docs.concat(regionDocs);
+    }
+  };
+};
+
+function createRegionDoc(folder, relativePath, regionName, regionContents) {
+  const path = folder + '/' + relativePath;
+  const id = path + '#' + regionName
+  return {
+    docType: 'example-region',
+    path: path,
+    name: regionName,
+    id: id,
+    aliases: [id],
+    contents: regionContents
+  };
+}