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/rendering/indentForMarkdown.js

@@ -0,0 +1,62 @@
+module.exports = function(encodeCodeBlock) {
+  // var MIXIN_PATTERN = /\S*\+\S*\(.*/;
+  return {
+    name: 'indentForMarkdown',
+    process: function(str, width) {
+      if (str == null || str.length === 0) {
+        return '';
+      }
+      width = width || 4;
+
+      var lines = str.split('\n');
+      var newLines = [];
+      var sp = spaces(width);
+      var spMixin = spaces(width - 2);
+      var isAfterMarkdownTag = true;
+      lines.forEach(function(line) {
+        // indent lines that match mixin pattern by 2 less than specified width
+        if (line.indexOf('{@example') >= 0) {
+          if (isAfterMarkdownTag) {
+            // happens if example follows example
+            if (newLines.length > 0) {
+              newLines.pop();
+            } else {
+              // wierd case - first expression in str is an @example
+              // in this case the :marked appear above the str passed in,
+              // so we need to put 'something' into the markdown tag.
+              newLines.push(sp + '.');  // '.' is a dummy char
+            }
+          }
+          newLines.push(spMixin + line);
+          // after a mixin line we need to reenter markdown.
+          newLines.push(spMixin + ':marked');
+          isAfterMarkdownTag = true;
+        } else {
+          if ((!isAfterMarkdownTag) || (line.trim().length > 0)) {
+            newLines.push(sp + line);
+            isAfterMarkdownTag = false;
+          }
+        }
+      });
+      if (isAfterMarkdownTag) {
+        if (newLines.length > 0) {
+          // if last line is a markdown tag remove it.
+          newLines.pop();
+        }
+      }
+      // force character to be a newLine.
+      if (newLines.length > 0) newLines.push('');
+      var res = newLines.join('\n');
+      return res;
+    }
+  };
+
+  function spaces(n) {
+    var str = '';
+    for (var i = 0; i < n; i++) {
+      str += ' ';
+    }
+    return str;
+  };
+
+};