provision child documents for non-R Markdown files

[zkamvar]

@zkamvar and @tobyhodges this PR is still in progress but ran into a question about links. I added a link that links to the incubator that I think would be a reused link in other episodes. Is there a place we specify reused links like in the old template? Also are inline preferred or referenced a the bottom?

In the workbench documentation on episodes there is a learning objective about links but I don't see any info about links yet. Guessing that section is still in progress.

Originally posted by @sstevens2 in https://github.com/carpentries/lesson-development-training/pull/17#issuecomment-1044751544

As indicated by the questions in the comment, there is no clear way to include extra files and linking is not properly described.

Links

I had thought I described links, but the documentation is clearly not there. In short: links are relative to the project itself. So if you want to provide a link to the setup from an episode, you would use [link to the setup page](../learners/setup.md) and that will transform on build time to the appropriate link on the site.

Includes/common links

This is one place that does need work. We do not currently have support for a common links document. When I created {sandpaper}, I intended folks to use RMarkdown wholly (which would allow people to include child documents by using the child = 'file.Rmd' option in a chunk, but I quickly realised that it was an uncomfortable barrier for many who don't use R. However, I did not include an alternative yet. Whatever the alternative will be, it will be included in the lua filter for pandoc.