Improve R integration with new fastpages blog

[Gasta88]

From Frie:

work on improving R integration: there are some blog posts out there on this topic but this would require a deep dive into how the github actions of fastpages work and trying to expand them to work with output from rmarkdown and move the files where they’re supposed to be.

I think the priority would be:

1. make md with static plots work (e.g. work from this here: https://github.com/KonradUdoHannes/fastpages_tryout/blob/master/_posts/2020-01-07-rmarkdown_test.Rmd - you could copy paste that to the github repo)

2. make html output format work (that requires adding to the github action, i suppose) -> that should solve the interactive stuff

[pr130]

@Gasta88 input from Henry:

nbconvert is used in the end of the pipeline to transform the ipynb documents to html. fastai pages adds "only" the FrontMatter to the html documents which is then interpreted by jekyll. So I guess for R, the same should work. Export it with the tools available to html and add a header. The styling is then probably as you said just described by some css and html templates.

see https://correlaid.slack.com/archives/C05092DUH/p1612261020085400?thread_ts=1612198292.068200&cid=C05092DUH