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/content/examples/webpack/ts/config/webpack.common.js

@@ -0,0 +1,72 @@
+// #docregion
+var webpack = require('webpack');
+var HtmlWebpackPlugin = require('html-webpack-plugin');
+var ExtractTextPlugin = require('extract-text-webpack-plugin');
+var helpers = require('./helpers');
+
+module.exports = {
+  // #docregion entries
+  entry: {
+    'polyfills': './src/polyfills.ts',
+    'vendor': './src/vendor.ts',
+    'app': './src/main.ts'
+  },
+  // #enddocregion
+
+  // #docregion resolve
+  resolve: {
+    extensions: ['.ts', '.js']
+  },
+  // #enddocregion resolve
+
+  // #docregion loaders
+  module: {
+    rules: [
+      {
+        test: /\.ts$/,
+        loaders: ['awesome-typescript-loader', 'angular2-template-loader']
+      },
+      {
+        test: /\.html$/,
+        loader: 'html-loader'
+      },
+      {
+        test: /\.(png|jpe?g|gif|svg|woff|woff2|ttf|eot|ico)$/,
+        loader: 'file-loader?name=assets/[name].[hash].[ext]'
+      },
+      {
+        test: /\.css$/,
+        exclude: helpers.root('src', 'app'),
+        loader: ExtractTextPlugin.extract({ fallbackLoader: 'style-loader', loader: 'css-loader?sourceMap' })
+      },
+      {
+        test: /\.css$/,
+        include: helpers.root('src', 'app'),
+        loader: 'raw-loader'
+      }
+    ]
+  },
+  // #enddocregion loaders
+
+  // #docregion plugins
+  plugins: [
+    // Workaround for angular/angular#11580
+    new webpack.ContextReplacementPlugin(
+      // The (\\|\/) piece accounts for path separators in *nix and Windows
+      /angular(\\|\/)core(\\|\/)(esm(\\|\/)src|src)(\\|\/)linker/,
+      helpers.root('./src'), // location of your src
+      {} // a map of your routes
+    ),
+
+    new webpack.optimize.CommonsChunkPlugin({
+      name: ['app', 'vendor', 'polyfills']
+    }),
+
+    new HtmlWebpackPlugin({
+      template: 'src/index.html'
+    })
+  ]
+  // #enddocregion plugins
+};
+// #enddocregion
+