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/angular-cli.json

@@ -0,0 +1,59 @@
+{
+  "project": {
+    "version": "1.0.0-beta.26",
+    "name": "site"
+  },
+  "apps": [
+    {
+      "root": "src",
+      "outDir": "dist",
+      "assets": [
+        "assets",
+        "favicon.ico"
+      ],
+      "index": "index.html",
+      "main": "main.ts",
+      "polyfills": "polyfills.ts",
+      "test": "test.ts",
+      "tsconfig": "tsconfig.json",
+      "prefix": "aio",
+      "styles": [
+        "styles.scss"
+      ],
+      "scripts": [
+
+      ],
+      "environments": {
+        "source": "environments/environment.ts",
+        "dev": "environments/environment.ts",
+        "prod": "environments/environment.prod.ts"
+      }
+    }
+  ],
+  "e2e": {
+    "protractor": {
+      "config": "./protractor.conf.js"
+    }
+  },
+  "test": {
+    "karma": {
+      "config": "./karma.conf.js"
+    }
+  },
+  "defaults": {
+    "styleExt": "css",
+    "prefixInterfaces": false,
+    "inline": {
+      "style": false,
+      "template": false
+    },
+    "spec": {
+      "class": false,
+      "component": true,
+      "directive": true,
+      "module": false,
+      "pipe": true,
+      "service": true
+    }
+  }
+}