Feature: handle nocite

[jdutant]

As mentioned in https://github.com/pandoc-ext/section-bibliographies/issues/6#issuecomment-14752948936#issuecomment-1475294893 the filter ignores the metadata field nocite. (And suppress-bibliography?)

I can see two valid uses of the metadata nocite:

• if there's a global biblio in addition to the section ones, place the nocite citations in this biblio only.
• if there are only the section-level bibliographies, insert the nocite in each of them. (Not sure why one would want that, but perhaps.)

A related issue is whether the filter should provide section-level nocites and how. One solution would be to use special Divs. (Tricky case: a nocite div is present at the top of a section level 1, but the filter is run asking for bibliographies at section level 2.)