Better documentation on converting D7 themes to Backdrop themes, blocks and layouts #3580
Description
Part of #3578
If you are using a custom theme, it is advised to switch to core Bartik before upgrading. That would get you a working Backdrop version of your old site and then you could start either adding a completely new theme or porting the one you disabled. Again, another thing we should be giving a hint about (instead of the user going forward with the upgrade keeping their D7 contrib/custom theme and discovering that they "broke their site" after the upgrade). Perhaps we should be doing all that on their behalf: disable all themes and switch to Basis or the Backdrop version of Bartik. Either way, at the end of the upgrade, there should be a hint/message explaining what was done and why. Perhaps also link to documentation/videos of how to port a D7 theme.
During the upgrade, we should be making educated guesses where possible/safe, but also provide options where applicable. For example, as shown during this video there is a "Node layout" layout created with node/% as path. That's really intuitive (yay!!) and makes it possible to bring across any pre-existing visibility conditions (although #2321 happened during the training test upgrade). But we gave no option to the user to select a layout template for example or to place the rest of the blocks. OK, they can tweak that after the upgrade and I think that that is the best way to go about it (makes the upgrade a single step thing), but we did not tell them anything about it. Perhaps a hint/message "The blocks you had on your old site have been added to the Node layout layout. They should have been placed in appropriate regions, but perhaps you would like to double-check [link-to-manage-blocks-page-of-the-node-layout]."