We noticed the rails gem increased in size between the beta and rc
versions, and the reason is that some guides images were updated but not
properly optimized, and one image changed from png to jpg, way bigger.

Converting the jpg image back to png, and using ImageOptim to optimize
all the images for web view, improves the situation [ci skip].

---

In current master/4-1-stable:

    508K  guides/assets/images/getting_started/

In v4.1.0.beta1:

    268K  guides/assets/images/getting_started/

In 4-0-stable

    328K  guides/assets/images/getting_started/

After the patch:

    260K  guides/assets/images/getting_started/

- Used Article model instead of Post as it may confuse with 'post'
  requests
- Tried to wrap the guide to 80 chars lines as much as possible.
- Removed unused image
- Fixes #13764

Compressed all images from the guide through ImageOptim which saves sometimes up to 74.6% in size [ci skip]

Revert "Compressed all images from the guide through ImageOptim which saves sometimes up to 74.6% in size [ci skip]"

Reason: this change can't really be done in docrails,
see http://weblog.rubyonrails.org/2012/3/7/what-is-docrails/

This reverts commit bc06e8d7.

Compressed all images from the guide through ImageOptim which saves sometimes up to 74.6% in size [ci skip]

section