Problem/Motivation
There are usages of book in demo_umami which need to be moved so the module can be deprecated.
$ grep -ri book core/profiles/demo_umami/ | awk -F: '{print $1}' | sort -u | nl
1 core/profiles/demo_umami/themes/umami/css/classy/components/book-navigation.css
2 core/profiles/demo_umami/themes/umami/templates/classy/content/book-node-export-html.html.twig
3 core/profiles/demo_umami/themes/umami/templates/classy/layout/book-export-html.html.twig
4 core/profiles/demo_umami/themes/umami/templates/classy/navigation/book-all-books-block.html.twig
5 core/profiles/demo_umami/themes/umami/templates/classy/navigation/book-navigation.html.twig
6 core/profiles/demo_umami/themes/umami/templates/classy/navigation/book-tree.html.twig
7 core/profiles/demo_umami/themes/umami/templates/content/node.html.twig
8 core/profiles/demo_umami/themes/umami/umami.libraries.yml
Similar CSS and templates should be removed from other core themes too.