Support pagination

It’s not clear to me that this is actually a good idea, and this
functionality may be reverted later. It adds a lot of complexity
despite still having significant drawbacks, and the alternative pattern
of just generating archives and later expanding them is a much safer
and more versatile solution.

However, the pattern of e·g needing a paginated feed of all the posts
in one’s blog suggests that something along these lines (backed by this
method or archiving~expanding) *should* be supported by default in
⛩️📰 书社. One alternative might be to add (e·g) a `@书社:expand`
attribute to <书社:archive> elements, tho how exactly this would be
tracked on the resulting archives is unclear, and it would be
restricted to producing folders (with exclusively archive contents)
that could not easily be mixed with other files.