processPandocBiblio: Honour nocite directives

[slotThe]

Adds support for pandocs nocite functionality by having an appropriate field in the metadata of the site:

nocite: |
  @citation1
  @citation2
  …

This adds pandoc-types as an explicit dependency, but it was already transient before that, so it's not really new. I also wrote some documentation (in a separate commit, but feel free to squash) so more people can understand how to actually use these functions.

[jaspervdj]

Looks good to me, thanks for adding the docs as well! Feel free to add a contribution by note in the CHANGELOG using your preferred name if you want.

[slotThe]

Thanks! I've done that now