Files do not get rendered if they are not included as book chapters

Copy/paste this in Bash to create all the necessary files:

cat << EOF > _quarto.yml
project:
  type: book
  render:
    - index.md
    - this_is_rendered.md
    - this_is_not_rendered.md

book:
  title: "demo"
  chapters:
    - index.md
    - this_is_rendered.md

format: html
EOF

cat << EOF > index.md
# Front page

Demo
EOF

cat << EOF > this_is_rendered.md
# This is rendered

It was specified in chapters and got rendered. 
EOF

cat << EOF > this_is_not_rendered.md
# This is not rendered

It was not specified in chapters and didn't get rendered
EOF

Then run quarto render, which will output to default _book directory.