Add blogpost for non divergent branches and improve docs

[majamassarini]

• Create a blog post for the new packit feature
• Rework the documentation creating a new page dedicated only to the non divergent git branches feature (as it has been done for the "building in side tags" feature)
• mention the Pagure issue before the fix is published: Koji builds not triggered in branches where a merge has been fast forward if there are multiple commits in the merge. packit-service#2537