nixos-render-docs: check book structure

text content in the toplevel file of a book will not render properly.
the first proper element will be a preface, part, or chapter anyway, and
those require includes to produce.

parts do not currently allow headings in the part file itself, but
that's mainly a renderer limitation. we can add support for headings in
part intros when we need them

in all other cases includes must be followed by either another include,
a heading, or end of file. text content could not be properly linked to
from a TOC without a preceding heading.

0 files changed, 0 insertions, 0 deletions