Action: Add gfm export

[cagix]

Add an additional build target to make the content available as GitHub markdown (gfm).

TODO:

• Extend the MakeDeps filter: declare images and files as dependencies for a Makefile target (incl. new Makefile variables)
• Extend the Makefile: new target using the new variables
• Provide a template for Pandoc GFM and resolve all metadata like quizzes, video, challenges, tldr.... into content before/after the actual content (definition of the structure of the content pages analogous to Hugo archetypes)
• Removal of TeX commands and other shenanigans
• Translate fenced divs like showme or tabs into suitable GFM structures, remove non-translatable divs (like Filter: Add tabs/tab div (conversion to Hugo shortcode) #167 for Hugo, but this time for GFM)
• Check whether the scaling of figures (inline, figures) works automatically (pandoc -t gfm)
• Enable the build in the GH action

fixes #160