This single stack exchange the best documentation yet I've found about upgrading Drupal 8 and it is varied and cluttered. With a security update of Civi planned for April, we need to provide a more consolidated way to provide upgrade instructions. I volunteer to help, but since I have not yet been successful upgrading my Composer-installed Civi on D8 (hosted by CiviHosting) myself I am not qualified to write it. I am currently looking for solutions and collaborators.