docs: Add images to illustrate OneFlow workflow

Source of the images:
- https://www.endoflineblog.com/img/oneflow/feature-branch-rebase-and-merge-final.png
- https://www.endoflineblog.com/img/oneflow/release-branch-merge-final.png
- https://www.endoflineblog.com/img/oneflow/hotfix-branch-merge-final.png

This images are from the official blog post of OneFlow workflow.

Author: Babilinx