ci: add bookdown demo

[umarcor]

Coming from rstudio/distill#152, I'm finding it difficult to link a bookdown project with rticles templates. This draft PR adds the following:

• A minimal bookdown demo.
• A Dockerfile, which includes R, bookdown, TinyTeX and rticles.
• A GitHub Actions workflow.

In the workflow:

• The docker image is built.
• The default HTML (gitbook), PDF and EPUB outputs are generated.
• Each rticles template is used to generate a different PDF.
• All the PDFs are uploaded as artifacts.

Hopefully, this will allow users to preview each template, before installing the tools. At the same time, it can be useful for maintainers to detect inconsistencies or regressions.

---

So far, I could successfully build 5 rticles templates only (see example workflow run at https://github.com/umarcor/rticles/runs/964602732?check_suite_focus=true):

• acs_article: success; but, not title, no 'References' heading
• ctex: success; but no 'References' heading
• elsevier_article: success; but no 'References' heading
• mnras_article: success; but no 'References' heading
• plos_article: success; but no 'References' heading

I guess that all the following are missing some LaTeX package. However, I'd expect TinyTeX to install them automatically.

• acm_article: ! LaTeX Error: File 'acm_proc_article-sp.cls' not found.
• aea_article: ! LaTeX Error: File 'AEA.cls' not found.
• agu_article: ! LaTeX Error: File 'agujournal2018.cls' not found.
• amq_article: ! LaTeX Error: File 'authorpackages.tex' not found.
• biometrics_article: ! LaTeX Error: File 'biom.cls' not found.
• copernicus_article: ! LaTeX Error: File 'copernicus.cls' not found.
• mdpi_article: ! LaTeX Error: File 'mdpi.cls' not found.
• oup_article: ! LaTeX Error: File 'ouparticle.cls' not found.
• peerj_article: ! LaTeX Error: File 'wlpeerj.cls' not found.
• pnas_article: ! LaTeX Error: File 'pnas-new.cls' not found.
• rjournal_article: ! LaTeX Error: File 'RJournal.sty' not found.
• rsos_article: ! LaTeX Error: File 'rsos.cls' not found.
• rss_article: ! LaTeX Error: File 'statsoc.cls' not found.
• sage_article: ! LaTeX Error: File 'sagej.cls' not found.
• sim_article: ! LaTeX Error: File 'WileyNJD-v2.cls' not found.
• springer_article: ! LaTeX Error: File 'svjour3.cls' not found.
• tf_article: ! LaTeX Error: File 'interact.cls' not found.

I think that the following are available in the development branch, but not in the installed version of rticles:

• arxiv_article: Error: 'arxiv_article' is not an exported object from 'namespace:rticles'
• bioinformatics_article: Error: 'bioinformatics_article' is not an exported object from 'namespace:rticles'

IEEE template seems to fail with colorlinks: true:

• ieee_article: ! LaTeX Error: Undefined color Magenta'.`

The remaining templates fail with errors that I don't understand:

• ams_article: Error: LaTeX failed to compile demo.tex.
• asa_article: I couldn't open style file agsm.bst
• frontiers_article: ! Undefined control sequence. l.89 \firstpage
• joss_article: formal argument "pandoc_args" matched by multiple actual arguments
• jss_article: ! Undefined control sequence. l.14 \Plainauthor

[umarcor]

Now, skeletons are built too. ctex, mnras_article and rjournal_article seem to fail.

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[cderv]

Preview of all articles format as been done in the pkgdown website - https://pkgs.rstudio.com/rticles/articles/examples.html