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

scripts/ci-lite/test_aio.sh

@@ -12,19 +12,24 @@ echo 'travis_fold:start:test.aio'
 # Setup environment
 cd `dirname $0`
 source ./env.sh
-cd ../..
 
 
 echo 'travis_fold:start:test.aio.lint'
-
 # Lint the code
-cd "`dirname $0`/../../angular.io"
+cd ../../aio
 yarn run lint
 cd -
-
 echo 'travis_fold:end:test.aio.lint'
 
 
+echo 'travis_fold:start:test.aio.doc-gen'
+# Lint the code
+cd ../../aio
+$(npm bin)/gulp docs
+cd -
+echo 'travis_fold:end:test.aio.doc-gen'
+
+
 echo 'travis_fold:start:test.aio.localChromeSetup'
 
 # Start local Chrome
@@ -36,24 +41,18 @@ echo 'travis_fold:end:test.aio.localChromeSetup'
 
 
 echo 'travis_fold:start:test.aio.unit'
-
 # Run unit tests
-cd "`dirname $0`/../../angular.io"
+cd ../../aio
 yarn test -- --single-run
 cd -
-
 echo 'travis_fold:end:test.aio.unit'
 
-
 echo 'travis_fold:start:test.aio.e2e'
-
 # Run e2e tests
-cd "`dirname $0`/../../angular.io"
+cd ../../aio
 yarn start &
 yarn run e2e
 cd -
-
 echo 'travis_fold:end:test.aio.e2e'
 
-
 echo 'travis_fold:end:test.aio'