These two functions are used to track the depth while building the tree, which is needed to produce correct links.
val enter : ?kind:kind -> string -> unit
make (body, children) calls "the page creator" to turn
body into an
[ `Html ] elt.
set_page_creator was not called, a default creator is used.
val open_details : bool Pervasives.ref
<details> tags should be opened by default or not.