Use GFM instead of kramdown for rendering markdown

[zickgraf]

The only markdown file rendered is README.md. The default renderer is kramdown, but I assume that most people write this file using GFM (GitHub Flavored Markdown).

Example: https://homalg-project.github.io/SubcategoriesForCAP/README.html This would not render correctly using kramdown, but renders correctly using GFM: https://github.com/homalg-project/SubcategoriesForCAP/commit/0509c8f9c10f465c76cd03b133a80be51d9fd11b

GitHub documentation: https://docs.github.com/en/github/working-with-github-pages/setting-a-markdown-processor-for-your-github-pages-site-using-jekyll

[fingolfin]

With this patch, I cannot build the site locally, which I consider crucial for testing purposes:

$ jekyll b
Configuration file: /Users/mhorn/Projekte/GAP/repos/GitHubPagesForGAP/_config.yml
            Source: /Users/mhorn/Projekte/GAP/repos/GitHubPagesForGAP
       Destination: /Users/mhorn/Projekte/GAP/repos/GitHubPagesForGAP/_site
 Incremental build: disabled. Enable with --incremental
      Generating...
Markdown processor: "GFM" is not a valid Markdown processor.
                    Available processors are: kramdown

  Liquid Exception: Invalid Markdown processor given: GFM in README.html
             ERROR: YOUR SITE COULD NOT BE BUILT:
...

[zickgraf]

Ah, I see. I have just pushed a version using https://github.com/github/jekyll-commonmark-ghpages, which is publicly available and actually seems to be an old version of what GitHub uses when setting GFM according to this issue: https://github.com/github/jekyll-commonmark-ghpages/issues/9.

However, this seems hard to maintain (the last activity in the repo is from January 2019, one has to set some additional options in _config.yml which might become out of sync with what GitHub uses, the installation needs a Gemfile, and nothing of this seems to be "official"), so I will close the PR.