This is an upgraded version of my previous script https://greasyfork.org/en/scripts/429371-youtube-toggle-related-button. It does not hide the live chat or playlist box, if any of them exists, just places the video description under it.

There is an other version without any button that just puts the description above the related section and both of them are visible:
https://greasyfork.org/en/scripts/438747-youtube-description-on-the-right-side-above-related-videos