Spawns from #4300

It is used exclusively in citeDB:

citeDB is used in two locations:

1. Building the SRS' bibliography (see last line):

   Drasil/code/drasil-docLang/lib/Drasil/DocumentLanguage.hs

   Lines 178 to 189 in 1ecf38a

   	-- | Helper for creating the different document sections.
   	mkSections :: System -> DocDesc -> [Section]
   	mkSections si dd = map doit dd
   	where
   	doit :: DocSection -> Section
   	doit TableOfContents = mkToC dd
   	doit (RefSec rs) = mkRefSec si dd rs
   	doit (IntroSec is) = mkIntroSec si is
   	doit (StkhldrSec sts) = mkStkhldrSec sts
   	doit (SSDSec ss) = mkSSDSec si ss
   	doit (AuxConstntSec acs) = mkAuxConsSec acs
   	doit Bibliography = mkBib (citeDB si)

2. Building a "Notebook" (see last line):

   Drasil/code/drasil-docLang/lib/Drasil/DocumentLanguage/Notebook.hs

   Lines 26 to 37 in 1ecf38a

   	-- | Helper for creating the notebook sections.
   	mkSections :: System -> LsnDesc -> [Section]
   	mkSections si = map doit
   	where
   	doit :: LsnChapter -> Section
   	doit (Intro i) = mkIntro i
   	doit (LearnObj lo) = mkLearnObj lo
   	doit (Review r) = mkReview r
   	doit (CaseProb cp) = mkCaseProb cp
   	doit (Example e) = mkExample e
   	doit (Smmry s) = mkSmmry s
   	doit BibSec = mkBib (citeDB si)

Following our status quo, we should scan the documents we generate before generating them for any/all citations referenced.

This is marginally better because it means that we will only display the references we actually use in the documents.