Link for "New Post" is added in Getting Started guide [ci skip]

- Earlier(line number 862) it is said to add <%= link_to 'New post', new_post_path %> on posts index page before table tag. But when posts index page html is shown, "New Post" link is missing.
So aaded "New Post" link in this commit
Merge pull request #13067 from sagarjunnarkar/getting_started

Link for "New Post" is added in Getting Started guide [ci skip]