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/services/cheatsheetItemParser.js

@@ -0,0 +1,125 @@
+/**
+ * @dgService
+ * @description
+ * Parse the text from a cheatsheetItem tag into a cheatsheet item object
+ * The text must contain a syntax block followed by zero or more bold matchers and finally a
+ * description
+ * The syntax block and bold matchers must be wrapped in backticks and be separated by pipes.
+ * For example
+ *
+ * ```
+ * `<div [ng-switch]="conditionExpression">
+ *   <template [ng-switch-when]="case1Exp">...</template>
+ *   <template ng-switch-when="case2LiteralString">...</template>
+ *   <template ng-switch-default>...</template>
+ * </div>`|`[ng-switch]`|`[ng-switch-when]`|`ng-switch-when`|`ng-switch-default`
+ * Conditionally swaps the contents of the div by selecting one of the embedded templates based on
+ * the current value of conditionExpression.
+ * ```
+ *
+ * will be parsed into
+ *
+ * ```
+ * {
+ *   syntax: '<div [ng-switch]="conditionExpression">\n'+
+ *           '  <template [ng-switch-when]="case1Exp">...</template>\n'+
+ *           '  <template ng-switch-when="case2LiteralString">...</template>\n'+
+ *           '  <template ng-switch-default>...</template>\n'+
+ *           '</div>',
+ *   bold: ['[ng-switch]', '[ng-switch-when]', 'ng-switch-when', 'ng-switch-default'],
+ *   description: 'Conditionally swaps the contents of the div by selecting one of the embedded
+ * templates based on the current value of conditionExpression.'
+ * }
+ * ```
+ */
+module.exports =
+    function cheatsheetItemParser(targetEnvironments) {
+  return function(text) {
+    var fields = getFields(text, ['syntax', 'description']);
+
+    var item = {syntax: '', bold: [], description: ''};
+
+    fields.forEach(function(field) {
+      if (!field.languages || targetEnvironments.someActive(field.languages)) {
+        switch (field.name) {
+          case 'syntax':
+            parseSyntax(field.value.trim());
+            break;
+          case 'description':
+            item.description = field.value.trim();
+            break;
+        }
+      }
+    });
+
+    return item;
+
+    function parseSyntax(text) {
+      var index = 0;
+
+      if (text.charAt(index) !== '`') throw new Error('item syntax must start with a backtick');
+
+      var start = index + 1;
+      index = text.indexOf('`', start);
+      if (index === -1) throw new Error('item syntax must end with a backtick');
+      item.syntax = text.substring(start, index);
+      start = index + 1;
+
+      // skip to next pipe
+      while (index < text.length && text.charAt(index) !== '|') index += 1;
+
+      while (text.charAt(start) === '|') {
+        start += 1;
+
+        // skip whitespace
+        while (start < text.length && /\s/.test(text.charAt(start))) start++;
+
+        if (text.charAt(start) !== '`') throw new Error('bold matcher must start with a backtick');
+
+        start += 1;
+        index = text.indexOf('`', start);
+        if (index === -1) throw new Error('bold matcher must end with a backtick');
+        item.bold.push(text.substring(start, index));
+        start = index + 1;
+      }
+
+      if (start !== text.length) {
+        throw new Error(
+            'syntax field must only contain a syntax code block and zero or more bold ' +
+            'matcher code blocks, delimited by pipes.\n' +
+            'Instead it was "' + text + '"');
+      }
+    }
+  };
+}
+
+
+function getFields(text, fieldNames) {
+  var FIELD_START = /^([^:(]+)\(?([^)]+)?\)?:$/;
+  var lines = text.split('\n');
+  var fields = [];
+  var field, line;
+  while (lines.length) {
+    line = lines.shift();
+    var match = FIELD_START.exec(line);
+    if (match && fieldNames.indexOf(match[1]) !== -1) {
+      // start new field
+      if (field) {
+        fields.push(field);
+      }
+      field = {name: match[1], languages: (match[2] && match[2].split(' ')), value: ''};
+    } else {
+      if (!field)
+        throw new Error(
+            'item must start with one of the following field specifiers:\n' +
+            fieldNames.map(function(field) { return field + ':'; }).join('\n') + '\n' +
+            'but instead it contained: "' + text + '"');
+      field.value += line + '\n';
+    }
+  }
+  if (field) {
+    fields.push(field);
+  }
+
+  return fields;
+}