Add missing dependency kramdown-parser-gfm to Gemfile

[sibocw]

The dependency kramdown-parser-gfm seems to be missing. Without it:

$ bundle exec jekyll serve

logger was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.
You can add logger to your Gemfile or gemspec to silence this warning.
/Users/myusername/.rubies/ruby-3.3.5/lib/ruby/3.3.0/json/common.rb:3: warning: ostruct was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.
You can add ostruct to your Gemfile or gemspec to silence this warning.
/Users/myusername/Projects/centrarium/vendor/bundle/ruby/3.3.0/gems/safe_yaml-1.0.5/lib/safe_yaml/load.rb:22: warning: base64 was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.4.0.
You can add base64 to your Gemfile or gemspec to silence this warning.
/Users/myusername/Projects/centrarium/vendor/bundle/ruby/3.3.0/gems/liquid-4.0.4/lib/liquid.rb:72: warning: bigdecimal was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.4.0.
You can add bigdecimal to your Gemfile or gemspec to silence this warning.
Configuration file: /Users/myusername/Projects/centrarium/_config.yml
            Source: /Users/myusername/Projects/centrarium
       Destination: /Users/myusername/Projects/centrarium/_site
 Incremental build: disabled. Enable with --incremental
      Generating...
         AutoPages: Disabled/Not configured in site.config.
        Pagination: Complete, processed 1 pagination page(s)
  Dependency Error: Yikes! It looks like you don't have kramdown-parser-gfm or one of its dependencies installed. In order to use Jekyll as currently configured, you'll need to install this gem. The full error message from Ruby is: 'cannot load such file -- kramdown-parser-gfm' If you run into trouble, you can find helpful resources at https://jekyllrb.com/help/!
  Conversion error: Jekyll::Converters::Markdown encountered an error while converting '_posts/2015-04-18-dummy.md':
                    kramdown-parser-gfm
             ERROR: YOUR SITE COULD NOT BE BUILT:
                    ------------------------------------
                    kramdown-parser-gfm