Need better instructions for rendering

[zkamvar]

The current instructions for rendering documents are without a lot of caveats that I've encountered:

1. Figures get placed into the content/post/<postname>_files/figure_<something> directory by default and require the contributor to manually move them to static/post/<postname>_files/figure_<something>

2. Pandoc 2.x (shipped with the preview version of Rstudio) has deprecated markdown_github and now uses gfm for rendering. I've fixed this in 61ac7b7 for the render_new_rmds_to_md(), but it currently means that we can't put the rendering information in the header and expect it to work for everyone.

3. Due to Hugo, relative paths for figure references will not work, resulting in an error if the document is rendered by clicking the "knit" button. I believe this can be circumvented by using knitr::include_graphics() with here() (i.e. https://github.com/zkamvar/dissertation/blob/master/chapters/00-introduction.Rmd#L157).

4. I've found myself using the following command a lot:

unlink("content/post/practical-stegen.md"); learn::render_new_rmds_to_md()

[zkamvar]

Update:

I've added the learn::rerender() function, which will allow users to render a single practical (or set of practicals based on a glob)