* Complete re layout the documentation on PPO.

* Add how to develop in cn. Also rearrange the demo location.

* Update the doc to use CDN

* Update installation guide.

* Update the doc setting to use layout.html template

* Update the documentations to fix some grammer error

Update the link in how_to_dev_backend_en.md to the full path. This should resolve the issue where the API doc not building. (#288)

* Adds backend / SDK dev doc

* Update per Varun and chanwei's comment