Add cross-referencing to all rticles

[etiennebr]

I think that it could be easier to use cross-referencing in rticles. References to tables, figures and equations are ubiquitous to scientific papers. bookdown provides a generic solution that, I think, is more likely to bring users to success from the start.

\begin{equation}
(\#eq:eq)
c = a \cdot b
\end{equation}
  
see \@ref(eq:eq)

Gives

@cderv suggested that maybe we could provide two possible default outputs in the skeletons. Something like:

output:
  rmarkdown::pdf_document:
    base_format: rticles::mdpi_article
  bookdown::pdf_book: 
    base_format: rticles::mdpi_article

It has the benefit that the default output does not require bookdown, and it shows that other outputs are possible. While cross-referencing would not work from the start, enabling it would be a matter of picking the next output in the list. If using bookdown as the default output is excluded, then I think this solution is pretty good.

Following #327, I believe that the solution might need to be applied to all skeletons.

[etiennebr]

From #327 (comment)

Make that

output:
  rticles::mdpi_article: default
  bookdown::pdf_book: 
    base_format: rticles::mdpi_article

[cderv]

Would a vignette in rticles explaining how to use bookdown features by modifying the yaml header be a good idea to better document this ?

[etiennebr]

I agree that a vignette would be useful since there are so many options from bookdown. I still believe that using bookdown::pdf_book as default would enlarge the pit of success, though.