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/target-package/services/targetEnvironments.js

@@ -0,0 +1,51 @@
+module.exports = function targetEnvironments() {
+  var _targets = Object.create(null);
+  var _activeCount = 0;
+
+  var checkAllowed = function(target) {
+    if (!(target in _targets)) {
+      throw new Error(
+          'Error accessing target "' + target + '". It is not in the list of allowed targets: ' +
+          Object.keys(_targets));
+    }
+  };
+
+  var updateActiveCount = function() {
+    _activeCount = 0;
+    for (target in _targets) {
+      if (_targets[target]) _activeCount++;
+    }
+  };
+
+  return {
+    addAllowed: function(target, isActive) {
+      _targets[target] = !!isActive;
+      updateActiveCount();
+    },
+    removeAllowed: function(target) {
+      delete _targets[target];
+      updateActiveCount();
+    },
+    activate: function(target) {
+      checkAllowed(target);
+      _targets[target] = true;
+      updateActiveCount();
+    },
+    deactivate: function(target) {
+      checkAllowed(target);
+      _targets[target] = false;
+      updateActiveCount();
+    },
+    isActive: function(target) {
+      checkAllowed(target);
+      return _targets[target];
+    },
+    hasActive: function() { return _activeCount > 0; },
+    someActive: function(targets) {
+      for (var i = 0, ii = targets.length; i < ii; i++) {
+        if (this.isActive(targets[i])) return true;
+      }
+      return false;
+    }
+  };
+};