Improve the manual backporting instructions

[webknjaz]

1. It seems like the markdown formatting is a bit off — the paragraphs probably need to be single-lined.
2. The guide does not include details on the PR creation so the inexperienced contributors sometimes forget to switch the PR base branch from main/master/devel/etc to the correct backport target.
   • Bonus points for including a link to the specific PR creation page with the base branch pre-selected, if that's possible (it's not possible to choose the incoming branch, though, because it may be coming from a random fork depending on who submits it).