Replace Post with Article in getting started guide [ci skip]

- 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
Merge pull request #13774 from prathamesh-sonpatki/post-to-article

Replace Post model with Article model in getting started guide [ci skip]