Transform citations with missing references

[goerz]

Following up from #39 (comment)

If there are missing references (citations with no corresponding entries in a bibliography, e.g. due to a missing key in the .bib file), makedocs will show errors like this:

[ Info: ExpandCitations
┌ Warning: Bibliography anchor for key 'Mills2014b' not found.
└ @ DocumenterCitations ~/.julia/packages/DocumenterCitations/f0eDy/src/citations.jl:456
[ Info: CrossReferences: building cross-references.
┌ Error: invalid local link/image: file does not exist in src/PALEOcopse.md
│   link =
│    @ast MarkdownAST.Link("@cite", "") do
│      MarkdownAST.Text("Mills2014b")
│    end
│    
└ @ Documenter ~/.julia/packages/Documenter/nGNwI/src/utilities/utilities.jl:44

The second error is probably more confusing than helpful (and definitely redundant). It would be better if during the ExpandCitations step, citations without a corresponding bibliography entry would be transformed to something that's no longer a markdown link. That way, it wouldn't get picked up by the later CrossReferences step.

Maybe just keeping the "link text" in square brackets and making it red or something would do.

[goerz]

I'm going to go with rendering the citations as question marks, the same way LaTeX does it:

This is being implemented as part of #6